I remember learning the tapestry technique Pick and Pick. I was trying to follow instructions in a book and found myself constantly making mistakes. The middle of the piece would look great, but my selvedges were riddled with errors. My "aha" moment came when I stopped for a moment and considered how lines in tapestry work and how that creates the Pick and Pick pattern.
The key to understanding Pick and Pick is the same key to understanding how tapestry weaving works at a very basic level. In tapestry, your warp threads are always covered. When you weave one pass through your weft, you have successfully covered HALF of your warp threads because you are weaving over and under warp threads as you go across. When you weave a pass going back the other way, you cover the other half of those warp threads. Therefore, two passes with your weft makes a complete line.
With Pick and Pick you weave one weft in one direction in one color and then another weft in that same direction in another color to make a line instead of weaving in one direction and then back in the other direction.