Philatelist William Stevenson assigned a letter name to each pattern of stamp grill, starting with the A grill. Stevenson gave the Z grill its name because he didn’t know when it was used. The letter Z has nothing to do with its appearance.
A Z grill has between 221 and 252 pyramid-shaped points that form a rectangular grid covering about a third of a stamp. The points in a Z grill have a horizontal rooftop that is not found in any other grills.