What is the Nap?
If you run your hand along the table, from the baulk end towards the top, you will feel that the cloth is smooth. However, if you do the same but in the opposite direction you will feel the roughness of the cloth. The fibres of the cloth point towards the top cushion; this is the nap.
With the Nap
When a ball is moving nearer the top cushion it is said to be with the nap. Right side on the cue ball will cause it to turn right, left side to turn left. The cue ball will push off (with the initial impact of the tip), come back on line, and then continue to turn in the direction that the side was imparted. The cue ball effectively follows an elongated C shape.
When using side with the nap, the cue ball will turn the way you expect it to. Shots that are 'just' not on can be made with side e.g. a cannon that is too wide for a standard thick half-ball, can often be achieved by using running side. Shots that are slightly too narrow can be played with check side. Shots such as the Long Jenny can be played with cushion side and the spinning cue ball will hug the cushion.
Against the Nap
When a ball is moving away from the top cushion it is said to be against the nap. The cue ball turns the opposite way against the Nap. Right side on the cue ball will cause it to turn left, left side to turn right. The cue ball will push off (with the initial impact of the tip), come back on line, and then start to turn opposite to the direction that the side was imparted. The cue ball effectively follows an elongated S shape.
It difficult to use side when playing against the nap as the cue ball will drift away from the way you would expect. Beware using side at all, when playing against the nap. However, it is possible for shots to be made that are not 'on' by drifting the cue ball using side. Shots such as a cushion run-through are much more difficult when played against the nap as the 'wrong' side needs to be used, as cushion side will cause the ball to turn away from the cushion.
A ball played directly across the nap without side will turn slightly towards the top cushion. It is easier therefore, to play a pot along the top cushion than the baulk cushion.
Across the Nap
With a spinning cue ball, the nearer the direction of travel to either up or down the table, the more the nap has an effect. However, even small components of travel up or down the table, will cause the nap to turn a spinning ball, and has a marked effect on how you need to play the shot.
Furthermore, a ball played directly across the nap without side will turn slightly towards the top cushion. It is easier therefore, to play a pot along the top cushion than the baulk cushion.
Amount of turn
The slower a spinning ball is travelling up/down the table, the more it will turn (for the same spin). Balls played with extreme pace turn very little.
The heavier the cloth (the more nap it has), the more a ball will turn.