Most of the time the cue ball goes where you point it. Sometimes it leaves the tip, travels for a bit, then bends off its line and ends up somewhere you did not aim. That bend is not random. It comes from how you held the cue and where you struck the ball.

Why does the cue ball curve?

A level cue through the center of the cue ball sends it straight. Raise the back of the cue and strike off-center, and the ball curves. That is the whole recipe.

Here is what is happening underneath. When the cue is level and you hit the middle, the spin you put on the ball lies on a flat axis, and the ball rolls forward. When you raise the back of the cue, you tilt that spin axis. Part of the spin now points down into the cloth instead of along the table. The cloth grips that downward spin and pushes the ball sideways from the line it started on. The ball leaves roughly where you aimed, then the gripping spin drags it into a curve. For more on how contact point alone changes a shot, see spin in pool.

Swerve or massé: what's the difference?

They are the same effect at two intensities, and the line between them is soft. A gentle version, with a small cue elevation and a slight bend in the path, is usually called swerve. The steep, dramatic version, with a near-vertical cue and a sharp curve, is a massé.

The steeper the cue and the harder the off-center hit, the tighter the curve. Swerve often happens almost by accident when you play with side spin and a slightly raised cue. A massé is the deliberate version, set up on purpose to hook the ball around a blocker. If you want the deeper version of that shot, read what is a massé shot.

Does cue elevation always curve the ball?

No. Raising the cue on its own does not bend anything. You also have to hit off-center. A raised cue through the dead center still drives the ball forward in a straight line.

The curve needs both pieces: the tilted axis from the elevation, and the off-center contact that actually puts spin on that tilted axis. Miss either one and you get a straight ball, just with a bit of extra dig into the cloth. The terms here, swerve, massé, side spin, contact point, all show up in the pool terms glossary if you want them in one place.

How RealPool produces the curve

In RealPool the curve is not a scripted animation. Nothing in the code says "bend the ball now." The curve falls out of the same strike math that handles every shot in the game.

When you take a shot, the cueStrike function turns your tip position and cue elevation into spin on the cue ball. Raise the cue and the spin axis tilts, exactly the real-world cause of the bend. From there the normal cloth friction takes over and the path curves on its own. The model is seeded from David Alciatore's published technical proofs on billiard physics (TP A.30), which is where a lot of these numbers come from. You can read more about that approach in the deterministic physics engine.

Because the curve comes out of the strike model itself, it behaves the same way every time you set up the same shot. The practice sandbox is the place to feel it: put a ball in the way, raise the cue, hit off-center, and watch where the path goes.

Common questions

Why does the cue ball curve?

A raised cue plus an off-center hit tilts the spin axis so part of the spin points down into the cloth. The cloth grips that spin and pushes the ball sideways from its starting line, so the path bends. A level cue through the center sends the ball straight.

What is the difference between swerve and massé?

They are the same effect at two intensities. A gentle version, with a small cue elevation and a slight bend, is usually called swerve. The steep, dramatic version, with a near-vertical cue and a sharp curve, is a massé.

Does raising the cue always curve the ball?

No. Elevation alone is not enough. You also have to hit the cue ball off-center. A raised cue through the center still sends the ball forward in a straight line. The curve needs both the tilt and the off-center contact.