Bank shots refer to any shot where a rail is used to help pocket an object ball. Some bank shots are hitting the object ball to a cushion first or using the cushion first and then hitting an object ball. Either way, when you can execute these shots you will have made a giant leap in your odds of winning.
The geometry of bank shots is pretty straightforward. The physics is not. So that is what we will focus on in this article. Every bank shot is a combination of geometry and physics but the physics introduces the most variables to the shot. Things like ball spin, slide, friction, and how far a ball sinks in to the rail all affect the outcome of a bank shot.
When shooting a bank shot center cue ball you would think the return angle equals the angle the ball went at the rail. This is not true. The forward roll of the ball always makes the ball move forward of the geometric line. Now shoot the same shot, hard and the ball will follow closer to the geometric line. This happens because it is struck hard enough that it does not roll before contacting the rail. It was sliding at the point of contact with rail and this keeps it much closer to the geometric line.
Transfer of english from the cue ball to an object ball can have a significant effect on a bank shot. For many years people said this was not possible. It is possible. 
make a seemingly unmakeable bank shot. When attempting this shot you need to alter the spot on the object ball you hit because of the natural throw that sidespin imparts on the object ball. You also have to be hitting a relatively soft shot so the side spin has a chance to work off the rail.
When banking an object ball that is close to the rail, you must keep in mind the object ball will be sliding when it contacts the rail. This makes it seem to come up short on the return angle after rail contact. To properly execute this shot you must strike the object ball thinner to compensate for it. Some shots will also need outside english or throw to get that return angle where you need it.
Sometimes when we hit an object ball it seems to travel much straighter than it should based on the contact point. This is caused by the friction between the 2 balls and for a split second the transfer of force through friction makes the object ball follow the direction the cue ball was traveling. This is a common mistake by most beginner caliber players and must be compensated for with a small amount of outside english. This is especially true on bank shots where the object ball is close to the rail, roughly within 6 inches.
Bank shots that you send the cue ball to a rail first before contacting an object ball are often called lag shots. These shots are a very useful shot under circumstances where you do not have a line of site on the object ball. Recovering from an opponents safety play makes the ability to make these shots really important. In many ways the geometric line of these shots is more predictable. That is because you can adjust the cue ball spin and speed to accomplish almost any return angle after rail contact. The diagrams show an amazing result that you can achieve with a simple lag shot.
Lag shots are often a better choice than hitting an object ball first. When you have a ball in front of a pocket it can increase your position play options by hitting a rail first and then pocketing the ball. A couple examples of that are shown here. Often a lag shot is the better choice to achieve the position needed on your next shot.
