British Standards Document
BS 21
Specification for pipe threads for tubes and fittings where pressure-tight joints are made on the threads (metric dimensions)

https://doi.org/10.3403/00141824U

BS 21 is maintained by ISE/110. The current release of this standard is:

BS 21:1985 Specification for pipe threads for tubes and fittings where pressure-tight joints are made on the threads (metric dimensions)

https://doi.org/10.3403/00141824 (published 30/09/1985)

This standard is available from the following sources:

Other historical versions of this standard document also exist:

BS 21:1938 [current until 31/12/1957]
BS 21:1909 [current until 01/11/1938]
BS 21:1905 [current until 01/11/1909]