Along lines known as nodal lines , the peaks of the light waves from one slit and the valleys of the light waves from the other slit overlap and cancel each other. No light is detected along a nodal line. In the black radiating lines of the moire pattern, the black lines of one moire pattern fill the transparent lines of the other. Note that as the patterns are moved apart, the dark, nodal lines move together. This is the same thing that happens when light passes through two slits and the slits are moved farther apart.
Moire patterns magnify differences between two repetitive patterns.
If two patterns are exactly lined up, then no moire pattern appears. The slightest misalignment of two patterns will create a large-scale, easily visible moire pattern. As the misalignment increases, the lines of the moire pattern will appear thinner and closer together. Once you have learned to see moire patterns, you'll begin to see them practically everywhere. Look through two chain-link fences and notice the pattern.