Coil building is a handbuilding technique in which long, hand-rolled ropes of clay are stacked and joined one on top of the next, ring by ring, to build up a vessel's wall to whatever height or shape the coils are arranged into. It is one of the oldest pottery-forming methods and remains the standard way to build vessels larger or more irregular than a wheel can practically throw.
Clay is rolled by hand into even ropes, typically an even finger's thickness along their whole length (roughly half an inch to an inch across is common, thicker for a large or structural piece), and laid in a ring on top of a flat base or an already-built lower section. A hand-cranked or motorized extruder, which forces clay through a die, produces coils of a uniform diameter far faster and more consistently than rolling by hand, and is common in studios doing coil work at any real scale. Each new coil is joined to the one below with the standard handbuilding join, scored and slipped, then blended into the course beneath it by smoothing with the fingers or a rib tool, on the inside, the outside, or both depending on whether the coil texture is meant to show in the finished piece. Building continues course by course, with the lower rings left to stiffen somewhat between additions so they can support the weight of what is added above without slumping or buckling outward.
Because the wall goes up gradually and each course can be left to firm up before the next is added, coil building can reach heights and forms, including strongly overhanging or irregular profiles, that neither wheel throwing nor a single pinch pot could hold up while wet. Left unsmoothed, the visible ridges between coils are also a deliberate surface texture some potters use rather than something to always blend away. Because the whole vessel is built and joined by hand over an extended session, uneven drying between an older, stiffer lower section and a newer, wetter upper section is a common source of cracking, which is why coil-built work is typically covered with plastic between sessions to slow and even out drying.