A louvre can be pictured as a row of narrow strips stacked up a window frame, each one tipped so its outer edge sits lower than its inner edge. Look at it from outside and you see a solid wall of strips; look up from below and you can see straight through the gaps. That tilt does two jobs at once — air walks through, and rain slides off the outer edge instead of coming in. Everything on this page is either one of those strips or the whole thing made of them.