It's my understanding that TEX refers to strength and Wt refers to size. A TEX (strength) 21 is OK for piecing but might break. If that truely is the size-it's way to big for piecing. I prefer 50 or 60 wt and use a full 1/4" seam. If I'm desperate, I'll use 40 with a scant 1/4" seam.