Length of Pull
Also: LOP
The distance from the trigger to the back of the buttpad, fitted to the shooter so the rifle mounts square and the eye lands behind the scope.
Length of pull, abbreviated LOP, is measured from the face of the trigger to the rear of the buttpad, and it sets how far the rifle extends into the shoulder pocket. When it fits, the firing arm folds at a natural angle, the head settles into a relaxed cheek weld, and the eye arrives at the correct distance behind the scope without straining or craning. A stock that is too long or too short fights every one of those things.
Because shooters vary in build and because position changes between prone, bench, and a barricade, many modern stocks and chassis offer an adjustable LOP through spacers or a sliding buttpad. Getting it right is more than comfort: a square, consistent mount helps the stock track straight under recoil so the sight picture returns predictably for the next shot.