Ball first or cushion first?
A ball-first shot puts the cue ball straight onto the first object ball. A cushion-first shot, a bank, sends it into one or more cushions first. Banks look elegant, but they carry an error you can't see, and it grows the further the cushion is from the ball.
The hidden error in a bank
On a ball-first shot, you aim at something you can see: the object ball. On a bank you aim at a point on a cushion, and the ball has to come off that cushion at exactly the angle you expect. It won't, quite. Speed, spin, the cushion itself and the cloth all move the rebound by a little.
That little matters more with distance. A rebound one degree off becomes about 1.7 cm of error after a metre of travel, and a carom ball's whole contact window on a thin hit is only a few centimetres. Bank off a cushion right next to the first ball and the error hardly matters. Bank off the far end of the table and it can be the whole margin. Two cushions before the ball and the errors stack.
Numbers from the top level make the point. Across hundreds of cushion-first attempts by world-class players, about 45% scored, against roughly two thirds of their shots overall. Take out the easiest banks, the tickies, and the rate was closer to 40%.
When a bank is right
- Textbook patterns. Some cushion-first shots are played so often that players know them by feel: the ticky, the lag ticky and the umbrella. Their rebounds are short and familiar, so they're easy to judge.
- Kisses on the ball-first route. When the balls are close together, a direct hit can send the first ball back into the cue ball's path. A bank that approaches from another side can avoid it.
- Nothing simpler works. If no ball-first or single-cushion route has decent odds, a deeper bank is a fair choice; just know you're accepting a harder shot.
How the engine treats banks
The Carombo engine adds a rebound error to every cushion before the first ball, scaled by the distance from that cushion to the ball, and it penalises banks that aren't textbook patterns. It still offers banks so you can train them, but routes with two or more cushions before the ball only appear when nothing simpler has reasonable odds.