Source : Webster's Revised Unabridged Dictionary (1913)

Tail-bay \Tail"-bay`\, n.
   1. (Arch.) One of the joists which rest one end on the wall
      and the other on a girder; also, the space between a wall
      and the nearest girder of a floor. Cf. {Case-bay}.

   2. The part of a canal lock below the lower gates.