The formation of intermediate filaments within a cell is a multi-step process. Two monomeric subunits, interact side-by-side, or laterally to form a coiled-coil dimer. As two oppositely oriented polar dimers associate, they form a nonpolar tetramer, the basic unit of any intermediate filament. The tetramers then bind end-to-end to form a protofilament, which associates laterally with other protofilaments to form a protofibril, two to three nanometers wide. When four such protofibrils come together, they coil laterally to form a unit-length filament. Each unit-length filament is about sixty nanometers long and ten nanometers wide. These unit-length filaments further polymerize in a head-to-tail arrangement to form an intermediate filament.