Sipes are the small slits in the tread blocks of a tyre and are there to create a better contact between a tyre and wet road surface. M&S and winter tyres will, due to the nature of the work they are designed for, have a greater number of sipes on the tread surface than a standard road tyre.
A tyre with a greater number of sipes will usually be designed with smaller tread blocks to assist cooling.
There are different types of siping used in tyre design, some designed to lock together as the tread block moves. This locking together of the sipes allows the tread block to remain rigid (improving roadholding) whilst still retaining the intended benefit of sipes (wet grip). Another type of sipe is called a keyhole sipe (from Bridgestone). This sipe, as the name suggests is shaped so that the bottom of the sipe is wider than the top. The idea behind this is that as the tyre wears and the depth of the grooves and sipes is reduced, the sipe width is increased – therefore retaining a similar overall volume.
On some modern road tyres, sipes have been done away with altogether. Instead, a heavily-grooved tread design enhances wet grip.