The sides of an internal and external straight sided spline are straight and not curved as in an involute spline. There are shallow and deep standards, imperial and metric. They vary from 4 splines, equally spaced around the periphery of a shaft or bore, to 6, 10 and 16. The standards vary between bottom and top fitting.
A use is to enable one member to slide up and down another member whilst still under driving load. They are also used as a permanent drive when an interference fit is selected.