The graphitic structure does exist in some high temperature heat treated carbon fibers, but the bulk of the material is in the turbostratic form, where the distance between layer planes is about 0.34 nm and the layers are not planar. Microfibrils are also present in carbon fibers. The dimension of microfibrils is between that of the basic structural unit and the texture observed in micrographs of fractured carbon fibers. In high strength PAN-based fibers, the pores initially provided channels for removal of volatile material formed during pyrolysis and may be open pores at reaction temperatures. The model is useful, however, in that it demonstrates how the fine structure of the precursor material is carried over into the carbon fiber. Most of the published work on fine structure has been conducted in academic laboratories with fibers supplied by various manufacturers, probably with little information provided on how the fibers were made.