What to Use for a Tabby Weft
I am weaving a small table runner using 3/2 pearl cotton for warp and weft (red and green in the warp, white in the weft). The draft (#479, page 129, from Carol Strickler's A Weaver's Book of 8-Shaft Patterns, which I've modified a little) calls for a tabby weft. Are there any real guidelines for what yarns to use as a tabby weft? My loom is threaded with a warp sett of 15 ends per inch.
As always, I appreciate your input and help.
"Tabby" is usually an adjective that identifies the weft that weaves a plain-weave ground cloth when there is also an alternating pattern weft, as with overshot and summer and winter. The tabby weft is usually the same yarn (size and color) as the warp. Usually, if you were to cut only the pattern weft out of an overshot or summer-and-winter cloth, the remaining fabric would be a balanced plain weave (the number of warp threads per inch equal to the number of weft threads per inch). Sometimes people call a plain-weave cloth a "tabby" or a "tabby cloth." I think that the word "tabby" is most useful in distinguishing between the two types of weft (tabby vs pattern) so I restrict its use to that.
The draft you are using from the Strickler book is an overshot draft. If you are using 3/2 pearl cotton for the warp, in theory, your tabby weft should be 3/2 pearl cotton in the same color (green/red). The pattern weft of an overshot fabric should be two or three times the thickness of the warp and tabby weft yarn. Your warp sett of 15 ends per inch would not allow a balanced plain weave with a 3/2 pearl cotton tabby weft PLUS a much thicker pattern weft. The total weft sett (counting both wefts) would need to be 30 picks per inch (15 pattern, 15 tabby) to square the design. Even if you used a finer yarn for the tabby weft (sometimes weavers do this in order to beat an overshot pattern to square), your warp sett is much too close and you'd need a different, thicker, pattern weft.
For this design in a cloth with an appropriate weight for a table runner, using 10/2 pearl cotton as warp and tabby weft and 3/2 pearl cotton as the pattern weft would have been your best choice. Since you already have this threaded and since the longest weft float in your draft is only over six threads, I'd see what happens if you omit the tabby weft altogether. You would be weaving a twill fabric instead of an overshot one, but a sett of 15 epi is about right for twill in 3/2 pearl cotton. Otherwise, you need to spread the warp sett probably to 10 epi and use a thick, soft white pattern weft with your green/red 3/2 cotton tabby weft.