1) Barrel groove 2) Groove in a barrel stave 
groove in the staves of a cask
• (n.) A cooper`s tool for making the grooves for the heads of casks, etc.; also, the groove itself.
Croze noun [ Confer Cross , and Crosier .] A cooper's tool for making the grooves for the heads of casks, etc.; also, the groove itself. 
the groove at either end of the staves of a barrel, cask, etc., into which the edge of the head fits. · a tool used by a cooper for cutting such a groove.