What do the little red diamond markings on a tape measure or rule signify?
If the marks are at 16 inches, 32 inches, 48 inches, etcetera,
those are the distances studs in a wall are supposed to be apart,
measured from the center of one stud to the center of the next one.
Wall studs are the vertical pieces of wood which the sheetrock is
nailed to. I think the marks indicate 1/2 meter increments.