Educative practice can be obtained on the baulk line by judging there the different contacts that are necessary in order to make pot strokes and the direction that is taken by the object ball as the result of such shots.
Place the cue ball on one of the end baulk spots, and the red ball on the other. Now aim dead straight at the object ball, which should come back along the line and kiss the cue ball back to the spot. It is not nearly so easy to make a perfectly accurate central stroke at short range as some may suppose. Next take aim midway between the centre and edge of the object ball and note the run of the two balls. Repeat this many times until the eye becomes familiarized with the three-quarter ball pot or run-through, in which the object ball proceeds along a line drawn through it a quarter of an inch from its centre as viewed when settling down to the stroke.
The third aim should be taken dead at the edge of the object ball and the same observation made of the run of the two balls. It will now be found that the object ball proceeds along an imaginary line drawn through it from midway between its edge and centre and this is what is called a half-ball pot. Yet another common aim in potting has to be practised and that is the quarter ball pot. The aim for this is half an inch wide of the object ball, which departs along a line drawn through it a quarter of an inch from its edge as viewed when settling down to the stroke.
There are also five other aims which are sometimes needed, especially at short range, and these are the grazing ball, the eighth, the three-eighth, the five-eighth, and the seven-eighth, the aims for which are severally an inch, three-quarters of an inch wide of the edge, and a quarter of an inch and three-quarters of an inch fuller than the edge. With these nine contacts almost every stroke that is required on a billiard table can be made by quarter inch progression, and it is comparatively rarely and chiefly at short range that the "eighth" contacts are needed. At long range it is better simply to aim a little fuller or finer than half-ball and a little finer than full ball for the five-eighth, three-eighth, and seven-eighth contacts without troubling as to more exact observation, which is, indeed, impossible.
Much of a professional's confidence in his strokes arises from his knowing exactly what he is aiming at and what will be the precise result of his stroke if execution should harmonize with intention. He may say (as most professionals do) that he has no belief in theory in billiards but he is illustrating theory in every stroke that he makes. Hemay never have studied theory in his life, but he has simply travelled by a longer road to the same goal as that to which theory, combined with practice, would have led him.
Billiards is an exact science and it is governed from first to last by theory There is not a stroke upon the board that could not be executed and controlled by mechanism and accomplished in this way every time There is a certain definite and fixed line of aim to be taken, at a definite and fixed angle, and with a definite and fixed degree of top, bottom, or side, or combination of side with either of the other two methods of compensation. All that articles on billiards can do is to indicate to the student when and how these aims and compensations are to be applied and leave him to practice until proficient upon the principles thus indicated.
The true half-ball contact sends the two balls on their way at fixed angles and if, in a series of apparently identical in-offs the object ball does not strike the same cushion in the same place or follow a line that would lead to that result, the shot is not a half-ball one, although the immediate score may be made every time.