Intermediate filaments are so called because they are intermediate in diameter between microfilaments and microtubules, measuring about 8-10 nm. Whereas microfilaments and microtubules are both assembled from a single type of protein (actin and tubulin, respectively), there are several types of intermediate filament protein. You are probably most familiar with the type called keratins, which are abundant in the cells at the surface of mammalian skin. An intermediate filament contains about eight 'protofilaments' wound around each other in a rope-like structure (Figure 8c) which confers great strength. Unlike actin filaments and microtubules, the intermediate filaments are not involved in cell movements; their main role is to provide mechanical strength to cells and tissues.