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.
http://it.geocities.com/mp_pollett/servian.htm (see the great cross-section
of the Servian Wall in Piazzale Albania in Rome showing headers
and stretchers clearly.)
http://www.livius.org/a/italy/rome/servianwall/servianwall.jpg (Servian
Wall near Stazione Termini--a little hard to parse because of weathering.)
http://www.quikrete.com/diy/StoneMasonry.html (The Quickrete company offers
their advice on ashlar construction with more complex shapes.)
http://www.masonryinstitute.com/guide/part3/systems_b1_pg1.html (The Northwest
Masonry Guide offering information on complex bond patterns, illustrating varied
courses of headers and stretchers.)
http://www.sidesways.com/fllw/hanna.php (Wright's Hanna House, illustrating finger bonds at the corners.)