Meaning of bull header

bull' head"er

Pronunciation: [key]
— Masonry. Masonry.
  1. Also calleda brick having one of the edges across its width rounded for laying as a header in a sill or the like.
  2. a brick laid on edge as a header, as in a rowlock. Cf. bull stretcher.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.