@SeraFaith3 these types of "zigzag" patterns always have a tendency to get wavy, especially when segment-knotted on a diagonal. You could try knotting straight across, or add a border string to each side to give the edges a consistent knot structure.
Another work-around I've tried (if you have a foam macrame board or something similar) is to use a pin and stick it in between the knots at the points where the pattern tends to "suck inward" (like at the center of the triangles) and tug it outward and pin it in place so when you knot the following rows it stays put. So for string E, it's not used very often and can accidentally get pulled through the other knots, creating tension and pulling the sides inward at row 7, 21 and 28. The black strings at rows 13-15 could also do this if they get pulled too tight. Just make sure those areas have breathing room to stay out to the edge. 😊
Amollie
