Sett was used in Middle English in the sense of 'fixed in place'. Is has since dropped the second 't' to become simply set in almost every case, except the following:
Badger sett -- a badger's burrow. Setts range from a single tunnel with a wider chamber at the end to large networks of interconnected tunnels and chambers. It's not uncommon to find setts with dozens of entrances, and they may have a dozen or more badgers living in them.
If you suspect that you have a badger sett in your area, look for a few key clues: sett tunnels are flat-bottomed with an arched top, looking like a 'D' lying on its back. They usually have heaps of excavated dirt in front of the burrow, and there are often clumps of dried grass, leaves, and bracken left lying in front of the burrow (the badgers collect these for bedding). Badger setts are larger than fox earths and rabbit warrens, about 20+ cm across, and the earths and warrens are rounder (fox earths are actually often taller than they are wide). Foxes and rabbits may take over abandoned setts (or even share occupied setts with badgers), which will usually be evidenced by droppings around the entrance of the sett.
The Protection of Badgers Act (1992) protects badgers and badger setts in the UK. It is illegal to destroy a sett, to obstruct access to any entrance, or to disturb a badger within the sett. You may need a licence from English Nature before undertaking any significant construction project within 30 meters of the sett, and 'a sett' is defined as ‘any structure or place that displays signs indicating current use by a badger’.
Tartan sett -- one section of a tartan pattern. A sett is a very specific count of threads (for example, 4 black, 32 red, 32 black, etc.) The meeting between setts is called the pivot, after which the sett repeats itself in mirror image to form a complete repeat. (Although there are also asymmetrical setts that do not pivot and repeat the pattern). The pattern repeats itself over and over again, in both the warp and the woof, crossing over itself to produce the distinctive plaid tartan designs. A sett can be registered with the Scottish Tartans Authority, making it an official tartan of a family, clan, cooperation, or geographical area.
Aside from being useful in identifying exactly which sort of Irishman you may be dealing with, the width of the sett is also important to tailors and seamstresses, who wish to make sure that the pleats, belt loops, and other add-ons to the basic kilt all match up in pattern.
Stone sett -- a small rectangular or cubic stone, looking somewhat like a rough brick, usually of granite, used as a paving stone. They are the angular and often sharp-cornered counterpart to a cobblestone. The flat edges make them better for driving on, allow the stones to sit more tightly together, and allow a more stable foundation. They are supposed to be good in high traffic areas, but only of it is low speed traffic. Some setts are only flat on a few edges, and do not make as good roads/paths/etc.
The great benefit of setts is that they can be dressed, transported, and lain down by very low tech means (an iron mallet will square them up, a wheelbarrow will carry them, and anyone can lay them down). I spent a couple of years living in a small country where 99% of paved roads were paved in setts, and the paved roads were certainly better than the dirt roads. They were, however, bumpy, prone to water and mud damage, often treacherous to walk on, and in constant need of maintenance. Very quaint, though, and much better than cobblestone.