Okay, Madelyn – This is SO basic you won’t believe it; and maybe there isn’t a tried and true one answer. Ever since I’ve been weaving, I’ve always used a tabby thread that was finer than the warp thread. A few weeks ago I was in a weaving shop to pick out a tabby thread for use in a new weaving, and they said a tabby should be the same thread as the warp. So, is there one way or two ways? Thanks!
When I first started weaving, most weavers did use a finer tabby weft yarn than the warp yarn for overshot. To square designs in overshot and summer and winter and other tied weaves, you really have to have the same number of tabby picks per inch as warp threads per inch. That means that there are actually twice as many total picks per inch as ends per inch. Thus the choice of the finer tabby weft.
However, I do not like the way the plain-weave areas of the cloth look with a finer tabby weft; they are less solid in contrast to pattern areas. So I always use the same size tabby weft as warp and weave with a very firm beat. Another option: When it’s hard to square a design, reduce the number of pattern picks in the larger blocks.
Above all, though, unless for some reason a person doesn’t like it that way, I think designs meant to be squared should be squared. And even if you don’t like it that way, other people will think you don’t know they are supposed to be square if yours aren’t.