Side-buttons are buttons that close a garment such as underwear, trousers, overalls, shorts or skirts at the side, rather than at the front. The side buttons can either be on one side only (common with skirts), or on both sides. The latter was a popular style for men's and ladies' drawers up to the 1920s. The side-buttons allowed the rear of the drawers to be let down like a flap (see also butt flap), which was convenient for toilet use and also for spanking.
An alternative to side-buttons was a vertical slit at the rear, called "open style", but this variant existed only in female underwear.
Side-button drawers are featured in many examples of vintage spanking art, for example the work of Richard Hegemann. Side-button shorts are featured in one chapter of the spanking novel The Old Rectory; the "Dame" finds them convenient for spanking because they allow undoing the boy's shorts while he is already positioned OTK.