As per IEC98-1964:The reproducing stylus tip motion shall be tangent to or lie in a plane through the stylus tip and the record centre, preferably inclined at an angle of about 15degrees clockwise to the normal to the record surface through the stylus tip, as viewed from the record centre. The following drawing based on IEC98-1987 shows VTA as an angle of 20-25degrees anticlockwise as viewed towards the record centre.
Note on this drawing: In original IEC fig.1 tip was in the groove and not on the record surface.  Any cantilever shall be long enough as compared with groove depth. Thus the change of cantilever angle during modulation or tracing can be almost negligible irrespective of tip location on the groove: if maximum depth difference during modulation is approx. 50 micron and cantilever length is average 8mm, then ASIN(0.05/8)=0.00625 radian=0.36degree only . 
My comment on SRA: IEC98-1987 Art. 8.1 recommends a rake angle (SRA) of between 0 and -5degrees as optimum for reproducing styli while IEC98-1987 Art. 11.3.3 (non spherical tip) allows +4 and -8 degrees for SRA.  Before 1963 VTA/VMA was not stipulated for stereo groove. In the same way, SRA of the special form needle was discussed only after 1980. Analog is always fuzzy like pudding - you can see how some standards are made after representative products at the time.

The outlines of VTA & SRA

VTA and SRA can be changed with VTF. The following calculation is based on static vertical compliance. As per IEC98-1987(11.3.3 Stylus Rake for non-spherical tips):The stylus rake in the direction of groove travel shall lie between +4 and -8degrees when playing at the tracking force recommended by the manufacturer. This should be taken as allowance while optimum rake angle is between 0 and -5degrees as per Art.8.1.

The following graph shows the displacement images of stylus with VTA variations when tracing on sinusoidal vertical modulated groove. This drawing is originally made for lateral tracking error. When this drawing is seen from horizontal level, then this drawing can show the relation between VMA and VTA. y(VTA=VMA), y'(VTA<VMA) and y"(VTA>VMA). The nature of this second harmonic distortion is same as that of lateral tracking error. The difference if any from lateral tracking error shall be: this distortion in effect is varying in accordance with (not only groove radius and recorded velocity, but also) the inclination of wave forms modulated in groove - because the spring back action of lacquer is non-linear and VMA in effect shall vary according to the momentary inclination of wave forms (hence VMA in actual record is not one and same throughout). Better forget about this complicated VTA/VMA matter! Nevertheless (Trotzdem) I enjoy vinyl. During making this simulation I understand why distortion due to tracking angle error is not easily audible. This distortion is eminent at the high velocity level and inner groove radius. Its distortion is usually inaudible at the lower recorded level than 5cm/s and angle difference less than 10 degrees [estimated maximum distortion around 4.2% at groove radius 6cm]. To attain the distortion indicated as following graph having parameters for instance : Displacement (amplitude) 25 micron, Frequency of modulation 1000 cycle/sec, Velocity 2*pi*f*amplitude =15.7 cm/sec, Groove Radius 5.75cm, VTA difference 0.4 radian=22.9degree and 2nd harmonic distortion 33.1% as calculated with usual equation [ (v*TAN(A)/V) where A is error angle in radian] or 42% if peak velocity of distortion component (y'-y or y"-y)  having double cycle mainly is compared to the velocity of fundamental undistorted sine wave (y).  v*TAN(A)/V can be rewritten as TAN(peak groove inclination of original wave form)*TAN(error angle) because signal velocity v is determined by TAN(peak groove incilnation)*V.

Do not care much about VTA and Lateral Tracking Angle errors since vinyl distortions from other factors are of higher grade. Consultants of Shure Brothers experimented and reported (Psychoacoustics, the Determining Factor in Stereo Disc Distortion JAES Volume 12 Issue 2 pp. 115-123; April 1964): "It was determined that even when the tracking angle error is reduced to zero, the distortion products remaining are, by established standards, unreasonably high. Listener tests conducted under carefully controlled conditions indicate that the ear and nervous system will effectively reject these remaining distortion products. "

The following tables are based on Japanese reference book "Record Player" (1971) by Mr. Yamamoto who reported VTA as follows:

Average is similar but variation range of VTA is narrowed after '70s. I will add the following table to above in order to show that variation range of VTA is further narrowed. The data of VTA are taken from catalogues and/or my approximate measurements. It is quite unsure that some longtime selling models have not changed their VTA for these three decades. Ortofon SPU Classic series reissued after 1987 are said to have VTA 20degrees while old SPUs in '70s were said to have VTA 15degrees nominally. I suspect that many manufacturers have been adjusting the nominal data in accordance with standard of each time without any essential change in product since the quality control of VTA especially for high compliance cartridges is very difficult. 

Sometimes deviation of +/-5 degrees from nominal VTA might be found in actual products. In case of EMT stereophonic cartridges around 1997, VTA 18 or 23degrees (each with allowance +/-3degrees) was selectable on demand. Interestingly I find the following passage in the export catalog [August 1973] of Neumann SX-74 cutter head : "...the cutter was constructed to cut an 18°angle when correctly mounted. Lacquer springback is included in this calculation."  This shall mean that cutter head angle is more than 18degrees and the vertical angle (VMA) recorded is around 18degrees compromising between old standard 15degrees and then current standard 20degrees+/-5 (IEC98A? or BS1928 in 1972). 

Various devices to change VTA drastically and effectively have been invented: for instance headshell or cartridge incorporated with adjustable VTA (some of Perpetuum-Ebner headshell & ADC cartridge as shown in US patents). They are theoretically good but practically nonsense since nobody can fix correct VTA for every cartridge whose VTA in actual production and practice is deviating from nominal value. Please note that true horizontal level at user end is not easily attained and that an additional flexible joint at shell may invite resonances due to mechanical weakness at joint. Also note that an invention by Hagenah for PE was originally intended to compensate the height difference between the first playing record and the last playing record in automatic record changers [the height of the stack of records more than 20mm - which was no problem for monaural records, but deviating from then current standard tracking angle 15degrees for stereo records].

There was an interesting patent in this connection (US patent 3538266 invented by Cho and assigned to Micro Seiki and embodied in their IM type cartridges VF-3100/5 & VF-3100/e around 1968). Unique stylus holder knob was intended for optimum stylus pressure. It is explained as follows: "In this position (Fig.7) the stylus pressure against the record 74 is zero (0) and the damper is, not being in the neutral position, deformed. The stylus pressure is progressively increased until the forward end of projection 68 reaches the surface of the record 74 as shown in Fig. 8. The projection 68 is preliminarily adjusted in length so that the stylus pressure is of optimum value when the pickup cartridge is in its position as shown in Fig. 8. In this position, the axis of the armature 64 is parallel with the tubular guard and is coincident with the axis of the magnetic pole of the magnet 58. The damper 73 is, thus, not deformed and is its neutral position, assuring that the armature 64 is precisely oscillated according to mechanical oscillation of the stylus 66. After completing the operation for increasing the stylus pressure, the bridge 70 is turned upwardly about the axle 69 into a recess 75 in the housing 50. Reproduction operation of the record 74 is then carried out in this state. "
This device might be useful for obtaining optimum stylus pressure enabling the armature to set in the neutral position for linear push-pull action. Why this device is not adopted in later models? I suspect that the damper itself tends to change with the lapse of time and temperature so that the preliminary designed position  (i.e. VTA for cantilever) does not always coincide with the required value of stylus pressure in practice. VTA is one thing, Trackability with due stylus pressure is another thing, Distortion is again another matter involved - and the users spin their reviews. 

The following drawings are taken from US patent papers by B.B. Bauer assigned to CBS. Angle A is cutter angle, S is springback angle and B is recorded angle as a result (see Fig.3b).  B (VMA:effective vertical modulated angle)= A-S.    Fig. 2: Vertical tracking error angle D = C (VTA of pickup) -  B (VMA) . [My note: S in Fig.2 does not coincide with S in other figures. S in Fig. 2 does not stand for springback angle but "variance" between VTA and VMA. Hence S in Fig.2 should be better replaced by D as tracking error angle]     Fig. 4A shows an inserted wedge for cutter compensating the springback value. 

According to Mr. Yamamoto (1971):
CBS laboratory reported springback effect at lacquer recording - the effective vertical modulation angle was much smaller than cutter angle. American recording companies CBS & RCA used cutter angle 23degrees while European companies such as London/Decca used 0 degree. As a result VMA became 0 for CBS/RCA and -10degree for London/Decca. At that time the manufacturers of cartridges were perplexed how to set their VTA for their cartridges. IEC/RIAA recommended standardisation of VMA to 15degrees so that SPU/EMT/Shure at the time adopted VTA as 15degrees. Cutter angle was adjusted to achieve VMA 15degrees by inclining cutter angle further. In case of Westrex, original cutter angle was 23degrees and its VMA became 0-1 degree due to springback of 22 to 23degrees. Hence additional wedge of 14degrees was inserted to achieve VMA 15degrees as a result.
Mr. Howard also wrote in HFN 2/2004 about VTA/VMA and told same history. There Mr. Howard made an interesting summary about VMA of actual records as follows (besides possible VTA variation of actual cartridges:  from 18 to 30degrees) :
1) No record has a single well-defined slant angle - it is signal-dependent and also varies with the position across the record surface.
2)The variability in average slant angle across any but the most meagre record collection is likely to be anything up to 5 degrees even with modern LPs, and even larger with older pressings.

Another report is found at "The BAS Speaker" (the Boston Audio Society's newsletter) Volume 8, Number 4, January 1980 (P.31):
The question is complicated by the uncertainties in the technique of measurement. The conventional RCA and DIN test records can yield measured vertical tracking angles as much as 5 degrees higher than those obtained using either the geometrical method or the CBS STR-160 test record. According to Shure Brothers, "a variation of approximately 3 degrees is obtained from two different test bands on the DIN 45542 test records."
The last comment refers to the measured variations of VTA on DIN test record (complex tones of 1850Hz+3150Hz & 370Hz+630Hz). I don't know RCA test record number though J.G. Woodward of RCA Laboratories around 1963-1965 worked vigorously on this subject (esp. FIM measurement using 400Hz+4000Hz tones recorded at a 4:1 velocity ratio in the vertical direction?): "Techniques for Measuring the Vertical Tracking Angle of Stereophonic Phonograph Pickups" JAES July 1965 with summary "The record contains bands having vertical recorded angles between -12° and +44° in 4° increments. A second group of bands contains angles between 7.5° and 25° in 2.5° increments to permit more careful measurements in the vicinity of the standard 15° angle." BAS experimenters used similar RCA record arranged specially by RCA. CBS STR-160 recorded 400Hz (single frequency) signals with VMA from -6 degree to 43degrees (15 steps) and its measuring method as I understand is: 1. Pick up differential output from two channels    2. Measure the value of 2nd harmonic distortion through band pass filter.    3. The lowest distortion at specific VMA of a band should indicate corresponding VTA of a cartridge. Following example of measurement is taken from a book by JVC(1979) engineers who proposed another method since CBS test records are discontinued to be produced. This method is based on geometric view as shown in the right figure. But this method has difficult points: 1. The microscope must have extra long focus distance    2. The cantilever length must be known beforehand or pivot fulcrum location can be seen - then nobody but maker  can measure it before assembly!    3. Parallax from observing point must be compensated (by triangulation?) since actual vertical tracking angle at playing is little different from observed angle. Every test records (RCA/DIN/CBS) seem to have different modulations (frequencies/amplitude/vertical or 45degrees etc) so that the measured data can be different. Some test tones are not designed for VTA measurement proper: for example DIN 45542(1969) also recorded  (vertical modulation angle fixed at 15degree and recording L and R alternatively at 45degrees direction) 300Hz+3000Hz for 33rpm or nearly 400Hz+4000Hz for 45rpm for the test of nonlinear distortions (FIM) with stereo pick-ups. When FIM is measured lower than 1%, then VTA of measured cartridge is estimated near to then current 15degrees standard. Current valid IEC(1987) recommends VTA/VMA 20-25degrees for both records and cartridges, but does not indicate any definitive measuring method for VTA/VMA (any specific test record with 20-25degrees VMA has not been arranged officially as of today).

Obscurity about VMA/VTA/SRA issues (almost past as vinyl itself).
I don't know any real reason why SRA for reproducing stylus is not straight up while almost all cutting faces of lacquer cutting styli are made upright. Maybe -2degree as optimum SRA is concerned with the elasticity of vinyl when tracing with VTF (vinyl deformation) - some persons claim different SRAs such as +3~+10 to be optimum for some records. Then it shall depend on the material of vinyl (hard or soft). It is impossible to make negative degree of VTA for cartridges. Then early stereophonic records have never been played back properly. Only laser pick-up can do if equipped with VTA adjustment function. Or early Decca cartridge with V/L mode is more suitable (later Decca MarkV is said to have VTA 15degrees). It is incredible that the material of lacquer and its elasticity is unchanged - I assume that improved lacquer materials or copper plated DMM (Direct Metal Mastering) by Telefunken should have smaller springback (for example 5 degrees only) and that normal pressings after '70s should have VMA as designed between 15 and 25degrees. I find a following passage in a JVC book (1979) P.168-"VMA shall change with the conditions of cuttings such as type of lacquer disc etc. When condition changes, the test record for measuring VMA by cutting with rectangular signal (triangle as groove form) is produced and cutter angle is adjusted to reach specified value of VMA. " and P.168-"Since 1960s (stereophonic era) VMA 15-20degrees was standardised. Once IEC specified 15degrees but recently revised VMA to 15-25degrees in accordance with the tendency of the recent produced cartridges".  And now since DIN45547(1981) and IEC98(1987) VMA and VTA are again revised to 20-25degrees from previous 15-25degrees. The norm is not improving any odd product, but rather revised to accept the status in quo.

If you are further interested in VTA issue, you should look into US patent #4359768(1982) assigned to CBS about "Vertical tracking angle meter"  - reporting as "the average vertical angle of ten high quality pickups that were measured in the course of the study was 29degree., with angles of some as small as 22degree. and of others as large as 33degree". There is another US patent #4752921(1988) invented by (Dual-)Thomson engineer about "Record player with means for capturing recording and scanning angle" which I assume to be connected with VTA shell equipped on Dual CS5000, but the most important point of this invention (effective measurement of VMA/VTA on record and cartridge) has not been realised. The value and effect of "springback" on recording materials are not mentioned in both patent documents. Thus again vertical angles among cutter and record and cartridge remain fuzzy. Funny enough every patent claimer would tell his own/new story.

BTW: Consideration on arm height adjustment (which is not VTA adjustment in strict definition). The object of height adjustment at arm is simply to make cartridge mounting face parallel to record/turntable (the height of cartridge body is ranging usually between 15-20mm). Height adjustment +/-5mm with same VTF cannot change effective VTA/SRA on cartridge essentially (for example: max +/-1.3degrees at lateral effective length of arm 220mm). Meanwhile there is a funny tweak term "Tail down or up" of arm as often called by amateurs. Its geometric and dynamic difference if any might be a matter of static balancing arm (VTF by shifting counter balance) and not the cartridge nor record. 

What is apparent from above construction (when VTF is adjusted at horizontal level of wand and then the height of arm is changed at arm base so that the rear counterweight shaft and front wand are inclined from horizontal level): Effective VTF shall increase for tail down while VTF shall decrease for tail up according to the relation among pivot and center of gravity each for counter weight and pickup head (position g is rotated on axis p by tail up or tail down). 

There is another design of arm as under. I think the following explanation by AT is tricky. The stylus drag force is relayed to cantilever fulcrum of cartridge via stylus tip so that the resilience of elastic damper at cantilever fulcrum will make arm to move upwards irrespective of arm pivot height difference. Anyway the fact is not so simple: the cartridge compliance and the effective mass of arm system and the kinds of forces (frequency and amplitude) from the groove should be involved in determining which one (cantilever or arm wand) starts lifting.

Moreover VTA is much more than the inclined angle of stylus-pivot line from horizontal level. The vector of frictional force between stylus and groove is directed on horizontal level. The following table is based on my pseudo-science since it is based only on the coefficient of stylus drag between tip and record. The stylus drag forces are never constant. 

Practical considerations (common sense): there is no reliable tweak which can be generally applied on every cartridge/arm.

There are too many variations in the make of tip shape, damper, mechanical/geometric/physical construction of arm/cartridge. Frankly speaking products allowances are much wider and problematic than nominal specification or theoretical calculation.  Hence I can only suggest some commonplaces as under:

1. Mount the cartridge top face horizontal when playing the records. There is variation in record thickness (the recorded area). But you have no need to worry about thickness variation since the warp rate of actual records is often more than 0.8mm. 
2. Adjust the downforce of arm at cartridge tip as recommended for a cartridge. The recommended VTF for a cartridge has usually allowance +/-20% from the middle (optimum?) and practically +/-50% can be allowed for some cartridges. Too much deviation from the recommended VTF shall affect sound quality because the generating element (magnet or coil) shall be shifted from the centre of generating system (the location of push-pull linearity). Please imagine what shall occur on speaker (woofer) when amplifier output contains high residual DC voltage more than 0.2V: speaker coils are biased. 
3. AND forget VTA/SRA/VTF issues and reproducing system itself and enjoy the music as you like - the last is most important. 

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