Revolutions per minute is the vanity metric of modern tennis

Spin numbers get quoted like serve speeds even though the same figure produces wildly different shots depending on trajectory.

A broadcast graphic tells you a forehand carried three thousand revolutions per minute and the number lands with a thud of significance. It should not. Spin rate on its own is close to meaningless without the launch angle and the ball speed, because those three together determine the shape of the shot and any one of them can be traded off against the others.

Take two forehands with the same spin figure. One is hit hard and low with the spin barely enough to bring it down inside the line. The other is hit slower and higher, the spin doing much more work relative to the speed, landing short but kicking above shoulder height. Same number on the graphic, opposite tactical purposes, opposite problems for the receiver. If the graphic showed only one figure, it should be the ratio of spin to speed, and that is a number nobody puts on television because it does not sound impressive.

The other reason to be sceptical is that peak spin is not what you play with. Any player can produce a big number on a ball they have set up on, with time, from a comfortable height. What decides matches is what the spin does when the player is stretched, or rushed, or hitting on the rise, and there the figures collapse for everyone. A player whose spin degrades gracefully under pressure is more valuable than one with a higher peak and a cliff edge.

I suspect the fascination survives because spin is the last thing in tennis that still feels a bit mysterious. Serve speed was demystified decades ago. Everybody understands distance and pace. Rotation is invisible, it produces effects that look like magic to a spectator, and a number attached to it feels like a window into something hidden.

It is not a window. It is a single frame from a film.

Ask instead where the ball landed, how high it bounced, and what the opponent did next. That is three questions rather than one, and it is the only version that tells you anything.