On Center

"On center" specifies a measurement to the middle of an object -- a hole, a stud, a shelf pin -- rather than to one of its edges, and it matters because the two conventions give genuinely different numbers for the same layout. Shelf pin holes spaced "2 inches on center" means 2 inches from the middle of one hole to the middle of the next, not 2 inches of clear space between them; framing studs "16 inches on center" means the same thing for stud spacing.

Getting the convention wrong -- measuring edge-to-edge when a plan specifies on-center spacing, or vice versa -- throws off symmetric layouts by exactly the width of whatever's being spaced, which is why the distinction is worth being precise about rather than assuming.