The proliferation of plastic pre-formed / pre-sized shutters sometimes causes confusion to those wanting shutters for their windows. This is because buyers settle for what they think is the best look for the sizes offered. The common error towards larger makes it look like shutter size equals window-unit size. Correct shutter measure should match sash pair height and width, not the window unit size. This many times can be obtained by measuring from within the home. And, if the shutters are to be both operable and painted, undersize them by one half inch in overall height and width. It's as simple as that!