Ashlar is an architectural term describing a type of masonry. It is the most common type of masonry seen today, formed of courses (layers) of squared, usually brick-shaped stone blocks. It was the mode of laying stones par excellence among the Greeks and Romans, called by the latter opus quadratum, a term still in use in some quarters.

There is nothing very difficult or peculiar about ashlar construction in general, though it can get very complex in special circumstances such as in highly-formalized Greek temple construction. In the simplest situations, the smallest face of the stone will be square and about half the size of either of the other sides. These 1-1-2 blocks can be laid in a number of ways.

To understand how courses of stones are laid, one must understand the concept of headers and stretchers. A stretcher is laid with its long sides parallel to the direction of the wall; it 'stretches' along it. A header is laid perpendicular to the direction of the wall and only shows you its small face, 'head'-on. A common way to lay a wall is that employed in the so-called Servian Wall (c. 350 BC) in Rome. One course consists of stretchers; the one above that of headers, the one above that stretchers again, and so on. Each course of stretchers has two blocks side by side stretching along the direction of the wall, while the headers are laid across them, just covering the width of the two blocks underneath. It is also possible to alternate headers and stretchers in a single course.

Even the simplest shapes can be laid in complex patterns; two well-known traditional patterns are English bond and Flemish bond (see the URL below). The objective is to stagger the courses in such a way that there is never a combination such that a continuous series of vertical joints run up the wall, fatally weakening it.

Corners can be similarly simple or complex. If bricks at the corner interlock like your fingers do when you pray (but without bending the tips down to your knuckles), this is called (surprise!) a finger-bond. It is common, and is prominently exhibited in the masonry of Frank Lloyd Wright's 1937 Hanna House.

URLs. (see the great cross-section of the Servian Wall in Piazzale Albania in Rome showing headers and stretchers clearly.) (Servian Wall near Stazione Termini--a little hard to parse because of weathering.) (The Quickrete company offers their advice on ashlar construction with more complex shapes.) (The Northwest Masonry Guide offering information on complex bond patterns, illustrating varied courses of headers and stretchers.) (Wright's Hanna House, illustrating finger bonds at the corners.)