The Pentacon Six mount (commonly abbreviated to P6, or Б in Cyrillic) is a breech-lock bayonet lens mount originally used by several medium format single-lens reflex cameras from East Germany.
The lens mount was originally designed by Carl Zeiss Jena and Kamera Werkstätten (KW) for the Praktisix camera. KW later merged with several other companies to become VEB Pentacon; the Praktisix was improved and renamed the Pentacon Six. This camera was manufactured for a much longer time (almost 40 years) and exported widely; hence the lens mount became associated most with the Pentacon Six name, rather than the original Praktisix.