This is how we take the measurement of our lampshades:
We take the measurement of top and bottom from straight side to straight side and always at the longest side as shown above.
Vertical height: The height of the lampshade is the vertical height (except from pleated lampshades where we use the measurement of the side length):
Bulb peg / Bulb sizes:
at left = candle bulb in the middle = crown bulb at right = standard bulb
Here you see a classic square lampshade on a beautiful bronze table lamp.