Width -
Measure across the window from measure point to measure point in three places according to the measuring instructions.

Height -
Measure the window from top to bottom in three places according to the measuring instructions. Specify the desired panel height for café type shutters.

 
Square -
A window is considered square if the difference between the largest measurement and the smallest is no larger than 3/16", and the difference between the diagonal measurements is less than 3/16".