There is no single accepted method for revision sequencing, with different industries having different needs. Let’s collect those here.


A common, simple system is alpha-lettering (A, B, C…). ASME gives some guidance on allowable letters to avoid ambiguities 1:

  • Up to three alpha characters e.g. AAB.
  • Avoid I, O, Q, S, X, and Z.


  1. ASME Y14.35-2014 paragraph 5.2.