Measuring For a Horizontal Sliding Replacement Window
Before you measure, you will need to remove the screen and open the window. Always measure the opening at the smallest dimension. In other words, measure from the highest point or where the frame intrudes most into the opening. In the image below, you will see 2 typical sliding window frame styles. Whichever frame style you have, always get the 'D' dimension at the narrowest opening as shown.