One or two identical protofilaments are present. Two protofilaments are always seen in AD PHFs and SFs, as well as in CTE type I and CTE type II filaments. Both filament types are ultrastructural polymorphs. In Pick's disease (PiD) and CBD, single or double protofilaments are present. Zoomed-in schematics show amino acids involved in protofilament interfaces. Pink oval represents the additional density in the straight filament of Alzheimer's disease (AD SF).