A blind is a covering for a window which allows light to be kept out.

A blind or a curtin.

By convention the longer side is called the length, but it doesn't have to be.

Consider a rectangular window:

If you were to buy a blind for a window, the length of the blind would correspond to the drop from top to bottom of the window, and the width of the blind would correspond to the distance from side to side. If the window was a short, wide window the width of the blind would be longer than its length!

A blind is a covering for a window which allows light to be kept out.

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