To measure Daylight Opening (DLO), measure the
visible glass, from stop to stop.
To measure the Tip-to-Tip dimension, measure
the longest part of the surround (perimeter) portion of the grille,
on the top of the profile (the portion that faces outward from the
To measure the Underneath dimension, measure
the shortest part of the surround (perimeter) of the grille, on the
bottom of the profile (the portion that touches the glass).
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