Sandwich Rubber

A single layer of cellular rubber (known as the sponge) covered with a single outer layer of ordinary pimpled rubber (known as the topsheet), the thickness of the top sheet not being more than 2 mm, and the total thickness of sponge and topsheet no more than 4 mm.

The topsheet may be attached with the pimples facing inwards (inverted rubber) or outwards (pips out rubber).