SETT refers to the relationship between your warp yarn (the yarn dressing your loom) and your weft yarn (the yarn you weave with). The variables we are looking at are 1) the gauge of both yarns (warp and weft) as well as 2) Our ends-per-inch (EPI), ie. how far apart our warp yarns are on the loom. The finer your warp and weft yarns, and the closer together your warps are the more detail you can achieve in your tapestry design. The key is to make sure your warps are not so close together that you are unable to maintain weft-faced weaving.
Ends-per-inch measures how many warp threads are in a one-inch-horizontal area. These examples above show two extremes to illustrate how much less detail we can achieve when we divide our EPI in half and use two strands of the same weft yarn as opposed to one, comparatively. The above image shows how I have woven the same design twice, once with fingering weight yarn (Tough Love Sock by SweetGeorgia) at 10 ends-per-inch (bottom) and another using two strands of that same yarn at 5 ends-per-inch (top). My ability to create detail diminishes significantly.
Now, what happens when you change only one of those variables? These tiny two-inch samples above are both woven at 9 EPI but the sample on the left is woven with a 2-ply fingering weight yarn (Everlea Fingering) and the sample of the right is woven with a 4-ply worsted weight yarn (Everlea Worsted). The sample woven with the finer weight yarn can achieve far more detail even though the EPI hasn’t changed. Think of weft-faced cloth as being made up of pixels. Each pixel in the finer example was reduced in height by half!
For a more subtle example, here are two samples which are the same design woven twice. The one on the left is woven in a two-ply fingering (Everlea Fingering) at its most dense possible EPI, 12EPI, and the one on the right is woven with a four ply worsted (Everlea Worsted) at its densest possible EPI, 10EPI. We can see that the fingering weight yarn achieves a far more dainty and detailed version of the design. The two ply nature of the fingering weight yarn also lends itself to this as, just as with knitting, anything three ply or higher creates more defined stitches. Two ply yarns offer less definition. A two ply yarn used in tapestry has an effect where each pick of weaving (or line) seems to blend into the next, having a less defined nature than that of a three ply yarn.
While the SETT you work with is completely a matter of preference, my general rule when weaving small scale tapestry is to reduce both my yarn gauge and my EPI so my tapestries are detailed and fine, like my Transitions tapestry above, which is 11” wide and woven at 12EPI with fingering weight yarn.
Sampling is always a good idea, but don’t forget to take into consideration the fact that, as you design larger tapestries, your small samples for them will not represent the overall feel of the tapestry, especially when considering weaving with heavy gauge weft. I find my bulkier weight samples feel cumbersome in design, but when scaled up can be quite elegant. When weaving larger scale tapestry, I’m comfortable weaving at an EPI of as little as 4EPI with extra bulky weight yarn like my Jefferson Park IV tapestry above, which is 25” square.
The important thing is that you are happy with your design. So, for best ‘happiness’ results, be sure to follow through with making samples. Your samples are experimentations in employing different EPI with the same weft, as well as different weft using the same EPI. Have fun with it! I look forward to seeing your samples in the forums!