The photo above shows a "pick-glass," outlining a square inch of cloth. I took the photo with my iPhone, shooting through the lens of the pick glass, and the image is surprisingly clear.
(OK, I have to confess here that I always thought it was spelled "pic" -- until I looked up "pick-glass" on Google. Yes, it's true. Now I know.)
If you're thinking this is a useful device and you want one, you can find them on Amazon, of course, by searching for "counting glass" or "thread counting glass" or "pick counting glass." I'm sure they're used all the time in the textile industry. For weavers, it's a very handy device.
In the photo above, I have woven what looks like a 24 picks per inch on a warp sett at 24 warp ends per inch -- which would be a balanced weave, except that I'm using different weights of yarn in the warp and the weft. (The orange warp ends are 16/2 bamboo and the pale green weft yarns are silk/stainless steel at an unknown grist, something like 20,000 yards per pound.)
Also, this is double weave, so there are another 24 warp ends and 24 weft picks on the back side of the cloth. You can see some of the warp and weft peering out in lilac and turquoise, respectively. So we're really talking about 48 epi and 48 ppi total. I decided to use my pick glass to reassure myself that I wasn't beating the gossamer-fine weft too hard.
Because weavers are persnickety, right? And rightfully so, as we want our cloth to have a good hand and our weave structures to look just so, whether it's plain weave or satin or twill or whatever. For that reason, weaving patterns will often specify ends per inch and picks per inch along with the type of yarns to be used.
If we're not paying attention, we wind up with this: