The five measurements: Nominal, R.O., M.O., DLO, tip-to-tip

Five numbers describe one window, and mixing them up is how openings come out wrong. From smallest to largest they nest like this: DLO (the glass you actually see) sits inside the nominal/tip-to-tip frame, which drops into the R.O. (the framed hole, window + about 1/4" shim each side), which sits inside the M.O. (the masonry/block hole the buck boards frame). Get the call size straight, add the right amount for the opening, and every other number falls into place.

One opening, five numbers — always confirm WHICH measurement a schedule or supplier is quoting.

Nominal / call size: a "3060" = 36 x 60 (catalog size)
R.O. = window + ~1/4" shim each side = W+1/2", H+1/2"
M.O. from nominal: +3.5" · from R.O.: +3" · from M.O.: as-is
Finished hole: window + 1/2" total shim (36x60 → 36.5 x 60.5)
36x60 example: 39.5 x 63.5 masonry opening
Size order: DLO < nominal/tip-to-tip < R.O. < M.O.

NOMINAL / call size — the catalog number you order by. "3060" means 36" wide x 60" tall. It is roughly the frame size, not the hole.
R.O. (rough opening) — the framed hole the window drops into: window + about 1/4" shim each side (W+1/2", H+1/2"). On Stick Framed openings the window goes straight into this R.O. with no buck.
M.O. (masonry opening) — the block hole the buck boards frame in Block Framed work. From a nominal size add 3.5"; from a labeled R.O. add 3"; a labeled M.O. is used as-is. All three converge on the same number.
DLO (daylight opening) — the glass you actually see through, measured inside the frame. It is the smallest of the five.
TIP-TO-TIP / overall frame — outside-to-outside of the frame including any nail-fin or leg. This is what has to physically fit, so it matters most on Florida Flange / Replacement and Unequal Leg installs.

Pros

One window, five numbers — knowing which is which stops the most common ordering and cut-list mistakes
The relationship is fixed: DLO < nominal/tip-to-tip < R.O. < M.O., so any one number checks the others
Same conversions the calculator uses: nominal +3.5", R.O. +3", M.O. as-is all land on the same masonry opening

Watch-outs

Suppliers and schedules don't always label which number they gave you — when in doubt, confirm whether it's Nominal, R.O., or M.O. before cutting
Tip-to-tip can exceed the nominal size when a fin or leg is included, so on Florida Flange / Replacement and Unequal Leg units check the overall frame against the hole

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