Although much can be effected in billiards with plain strokes, there are other things that can only be accomplished by the use of side.
There are three kinds of side in billiards. There is ordinary side, in which the cue ball is struck on its horizontal centre; side with screw, in which the cue ball is struck below its centre; and side with top, in which the cue ball is struck above its centre. Striking the ball above its centre increases its forward rotation, and striking it below its centre imparts to it backward rotation, so that while these influences last a ball struck with side above its centre is deflected less, and one struck with side below its centre is deflected more, than one struck exactly on the horizontal centre.
The purposes of side are (1) to vary the angle at which the cue ball runs after striking the object ball or balls; (2) to vary the angle of rebound from a cushion; (3) to enter more easily a pocket which is not a very open one; and (4) to quicken or retard the run of the cue ball after it has struck a cushion.
When neither of the purposes that have just been described is to be effected by the use of side, it is much better to play a plain stroke. In making a cannon, for example— either ball to ball, or off a cushion, or by striking a cushion first—there is no justification for the use of side if, by simply playing for a fuller or finer plain ball contact the score can be as certainly made and as good a position ensured for the succeeding stroke.
In the same way, side is only required for an in-off when (1) the pocket is not sufficiently open, or (2) when it is desired to vary the run of the object ball towards such pocket, or (3) when, by use of check side, compensation for extra force can be introduced.
An extremely important point that is not universally known in connection with the use of side in billiards is that the side communicated to the cue ball does not operate in fast strokes until a cushion has been struck. It may seem to do, but this is simply the" forcing "effect of the stroke itself.
To prove this, place a ball on the centre spot of the table and play central strokes with top and top strokes with side; ordinary central strokes and central strokes with side; ordinary screw strokes and screw strokes with side. It will be found that the deflection of the cue ball after contact is not increased or decreased in either instance by the use of side until a cushion has been struck.
It is chiefly in slowly-played strokes that the employment of side becomes so conspicuously useful, but the player has to bear in mind that the deflection of the cue ball, due to the side with which it is laden, is a factor to be reckoned with both before it reaches the object ball and after it has parted from it.
This deflection is increased with the length of travel of the cue ball, and the aim must be varied accordingly. If slow running side is imparted to the cue ball and the stroke is a half-ball one at table length, aim should be taken almost full on to the object ball to allow for the variation in travel. If slow check side is used the aim should be almost an inch wide of the object ball. At half and quarter table length this allowance may be respectively halved and quartered, and down the table the whole thing must be reversed.