If you have a square lamp, a square lamp shade would often be the obvious choice, because it gives the lamp and shade a perfect harmony. Square lamp shades are popular and they look fantastic on many different lamps.
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):
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.