subprincipal

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An assistant school principal.
• noun An auxiliary or bracing rafter in a frame.
• noun Music An open diapason subbass in an organ.

from The Century Dictionary.

• noun An under-principal.
• noun In carpentry, an auxiliary rafter, or principal brace.
• noun In organ-building, a subbass of the open diapason class.