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The turntable arm plays an important role in the reproduction of vinyl records. It is the strategic bridge between the groove reader, the phonograph head, and the mechanics of the turntable platter. In some and quite expensive vinyl record players it is even separate from the platter mechanics. A good arm must be properly engineered, built with rigid materials capable of damping vibrations, its parts must have as little friction as possible but at the same time rigidity and must not introduce vibrations: the work that must be done is that to allow the head to "read" the micro-grooves of the discs following as much as possible the trajectory held by the engraving head.

The arms can be divided into two main categories: pivoted and tangential.
A good approximation of a disk is to think of it as a set of concentric circles: the ideal trajectory that a head should make coincides with one of the rays, this should be traveled without there being any changes in the position of the head, i.e. so that the the longitudinal axis of the head (precisely its stylus) is always perpendicular to the furrows. While the tangential arms satisfy (more or less) this requirement, the hinged arms introduce an error. Seeing the turntable from above, we can imagine the pivoted arm as a compass that describes an arc of a circle that crosses the disc from the edge towards the center. This means that the axis of the head is not tangent to the furrows and therefore that the stylus is not correctly positioned, therefore distortion occurs. Some construction features, such as the curvature of the arm barrel or the head holder shell (called offset angle) and the head overhang beyond the center of the disk (overhang), reduce the tangency error to less than a couple of degrees, which is not enough to make the distortion introduced practically unheard. The tangency error and the resulting distortion decrease with increasing effective length (the distance between the arm pivot and the stylus): normally this value varies from about 230 mm (9 inches) of most of the arms up to to over 300 mm in 12-inch SME.
The tangential arms, on the other hand, are not fixed to a point but move parallel to the radius of the disc, dragged on a straight guide, and in this their behavior they approach the ideal reading arm. Unfortunately, the construction difficulties are greater and the control of the tangential movement must be very accurate, this makes their construction rather complicated and expensive.

Apart from this, the two types of arm are distinguished by the fact that the pivoted arms necessarily require compensation, called anti-skating, of the centripetal force while the tangential ones are completely exempt from it. This occurs because the disc, due to the friction between the groove and the stylus, drags the arm forward but because with respect to the pin, the head is off axis towards the inside, curvature of the arm or of the head holder to make the offset) in this drag force there is a component that tends to bring the arm towards the center of the disc. This force must be balanced by another equal and opposite, because the stylus would push more forcefully on the internal grooves, rather than on the external ones, causing distortion, greater wear of the grooves themselves and the imbalance of the two stereo signals.
The compensation takes place through small counterweights, springs or magnets, all devices that go under the name of anti-skating. Since the force that drags the arm inwards depends on the friction between the stylus and the sulcus and this, in turn, depends on the reading weight, and on the shape of the stylus, the anti-skating force must be calculated as a function of these parameters even if in practice it is almost impossible to establish it with a formula. In doubtful cases it is therefore advisable to ignore the graduated scales and apply as much anti-skating as necessary to keep the arm still, or slightly moving towards the outside of the disc, when the stylus rests in the part without grooves on the external edge of the disc.
Another parameter to know is the Vertical Tracking Angle, or VTA, or the vertical angle with which the stylus must fit into the grooves. In principle it is sufficient to adjust the arm so that the barrel is parallel to the surface of the disc, even if there are exceptions, so you should not take anything for granted but consult both the instructions of the arm and those of the head. The undulations of the disk change the VTA quickly and dangerously, but the longer arms are less influenced; in this case the pivoted arms, normally longer than the tangential ones, have some advantage.

The arm is not resting on the disk rigidly, but there is a middle spring (the stylus and its suspension inside the head). Like all elastic systems, it has its own resonance frequency and damping characteristic. These quantities are defined by the elasticity characteristics of the head, its yielding and its reading weight, by the effective mass of the arm and the head. The effective mass is, simplifying, the mass that must be won to make the arm move by applying force at the point of anchorage of the head. Until a few years ago, a lot of importance was given to the fact that the resonance frequency of the arm-head system was between 8 and 13 Hz, the middle zone between the possible triggers due to disc ripples and the minimum audible frequency. This led to the construction of arms of very low mass (around 6 grams). An arm of medium mass (8-12 grams) will accept practically any head without problems, providing the best with those of medium-high compliance. In any case, rather than relying on theoretical reasoning or formulas, it is better to combine arms and heads based on their musical characteristics and rely on the experience of the best among the specialized shops. One thing to know before any purchase is the allowable head weight and the range of reading weights that you can apply. Although rare there are cases of incompatibility that are better prevented in order not to spend money badly.
As far as the compatibility with the turntable is concerned, some factors must be taken into account: The first one must be noted as to the physical possibility of mounting or not the arm you have chosen, especially for the ring roads. It is also important to note the possibility of closing the cover or possibly interposing spacers between the arm and the base in order to correctly adjust the VTA. Since not all arms have the same weight, it is certainly necessary to calibrate the springs of the floating frame turntables: a poor calibration of the springs can compromise the sound. Fortunately, many turntable manufacturers supply pre-drilled bases for different models of arms, coming in possession will save many tribulations. There are arms made of aluminum, steel, various metal alloys, carbon, even wood), internally damped, with straight, folded, double-folded barrels, decoupled counterweights, off-axis, etc. etc. Among all, a constructive feature of the pivoted arms may be of some importance in the choice is the type of articulation. There are many possibilities but it is known that some heads sound better if mounted on arms with uni-pivot articulation, that is, the type in which the arm is, so to speak, hovering on a single point, therefore free to roll around its axis and the heads that claim this type of articulation are those that have a very short, almost non-existent stylus.

To conclude a couple of practical tips. If you really can not help but mount the arm yourself, be very precise in complying with the directives provided by the manufacturers for installation on the turntable, but if you are inexperienced you should get help from an expert technician. Take even more care when assembling the head: buy or borrow a template for correct assembly, do not tighten the screws that hold the essay when the arm is already fixed to the turntable, stop it just enough so that it does not move on its own and tighten the screws only after removing the cartridge holder or the entire arm from the turntable. Once everything is assembled, observe the cantilever while reading a disc, if it oscillates excessively or if it is always curved inwards or outwards there is something wrong with the settings.
Be very demanding about it, a good editing can make the difference between an excellent sound and a poor performance.