How do you measure a window opening with ledges and gaps?

Glazing
Windows

Replacement windows are ordered to a hole size, and a hook that cannot sit flat against a sill, stool or return turns that order into a best guess. The T1M Utility's removable arm snaps on to give a flat registration face on an awkward surface, so the dimension is taken off the actual opening rather than approximated around it.

What the removable arm gives you

Arm snaps on for a flat registration face
Works where a hook cannot sit flat
Sills, stools and returns all register
A real dimension, not a best guess
The unit is ordered to hole size

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Use the removable arm. Windows are full of ledges, gaps and obstructions that a hook cannot sit against cleanly. The arm on the T1M Utility detaches and snaps into place to give you something to register against, so you get a real dimension off an awkward surface instead of a best guess.