The Expression number is taken from the full name as recorded at birth, and the system treats it as the account of a person's innate equipment. The Minor Expression is the same calculation run over the name the person actually goes by: a shortened form, a nickname that stuck, a married or trading name, a stage or pen name. All the letters are summed, vowels and consonants together, and reduced to one figure, with the master numbers eleven, twenty-two and thirty-three left unreduced.
The pair is meant to be read as a pair. Where the two figures agree, practitioners describe a person operating close to their given equipment. Where they differ, the difference is the reading: the Minor Expression is said to describe how the person presents and is met day to day, while the Expression continues to describe what they have to work with underneath. A woman who takes a married name, a writer who publishes under an invented one, and a man who has been called by his middle name since childhood are all cases the position was designed to catch.
The three minor positions in this chart, Minor Expression, Minor Personality and Minor Soul Urge, stand in exactly the same relation to their major counterparts and are computed the same way. They are the working name run through the arithmetic the birth name is run through, and read as an overlay rather than a replacement.