Part of the boundary wall slumped after the wet winter. The rule I was taught: every stone touches two below it, and the long side goes into the wall, not along it. Fill the middle with hearting as you go or the faces drift apart. It took a Saturday to take down the bad section and sort the stone into faces, throughs and fill, and a Sunday to put it back. The through-stones are what hold it; a wall without them is two thin walls leaning on each other.