Before the faces go up, the waller sets up the frames. These are two wooden frames shaped like the end view of the wall, wide at the bottom and narrow at the top, one at each end of the gap. They are pushed into the ground, made upright with a level, and lined up with the old wall on either side. Between them, string is stretched tight along each face, a few inches above the stones being laid. As the wall rises, the string is moved up the frames, course by course.
The frames show the batter, which is the inward lean of each face. A dry-stone wall is not built with vertical sides. Each face slopes in a little as it rises, so the wall is wider at the bottom than at the top. On the hill wall the top, under the copes, is about half as wide as the base. The batter gives the wall its strength. It keeps the weight of the stones bearing down and inward, toward the middle, and it means the faces lean on each other through the hearting instead of each standing on its own.
With the frames up, the faces are built in courses, a layer at a time, on both sides together. The waller picks up a stone, looks at it, turns it over, and sets it on the course below. The rule for placing it is short: one stone on two, and two stones on one. Each stone should sit across the joint between the two stones below it, the way bricks are laid, so that no joint runs straight up through more than one course. A long vertical joint is called a running joint, and it is a weakness. A wall full of them will come apart along those lines.
As in the footings, each stone goes in with its length running into the wall. Its outer face should be close to the line of the string, without touching it, and its top should be as flat as can be managed, tilted very slightly outward so that rain runs off. If a stone wobbles, a small wedge is put in from behind, never from the front, where it could work loose and drop out. Behind each stone, as soon as it is placed, the hearting is packed in by hand, so that the stone is held firmly from the back.
Choosing stones is most of the skill. A waller with a good eye seldom picks up a stone more than once. Someone learning picks up the same stone several times, tries it in a place, sees it does not fit, puts it down, and picks up another. Both are fine. Stones do not care how long it takes. Over a day, the hands learn which shapes go where. A long flat stone is a good course stone; a wedge-shaped stone is useful for filling a corner; a round stone is not much use in a face at all and goes into the hearting.
The stones get a little smaller as the wall goes up. The biggest are near the bottom, and the courses near the top use lighter stones, which also makes them easier to lift to that height. The joints get a little tighter too, because there is less weight above to hold things in place. By the middle of the second day, the gap is about knee high, level along its length, with both faces following their strings and the hearting packed tight between them.