How to Measure a Window's Daylight Opening with a Tape

A window's daylight opening is the visible glass gap within the sash; you need its size, among other things, to make a mosquito net.

The measurement is taken between the frame profiles near the sealing gasket: use a tape to measure the width and height of the opening at the top and bottom and record the readings.

This method gives the exact dimensions of the daylight opening without removing the unit and suits ordering a net or other made-to-measure parts.