The first and most important principle, that on which the whole of the
megalithic construction may be said to be based, is the use of the
orthostatic block, i.e. the block set up on its edge. It is clear that
in this way each block or slab is made to provide the maximum of wall
area at the expense of the thickness of the wall. Naturally, in
districts where the rock is of a slabby nature blocks of a more or less
uniform thickness lay ready to the builders' hand, and the appearance of
the structure was much more finished than it would be in places where
the rock had a less regular fracture or where shapeless boulders had to
be relied on. The orthostatic slabs were often deeply sunk into the
ground where this consisted of earth or soft rock; of the latter case
there are good examples at Stonehenge, where the rock is a soft chalk.
When the ground had an uneven surface of hard rock, the slabs were set
upright on it and small stones wedged in beneath them to make them stand
firm. Occasionally, as at Mnaidra and Hagiar Kim, a course of horizontal
blocks set at the foot of the uprights served to keep them more securely
in position. With the upright block technique went hand in hand the
roofing of narrow spaces by means of horizontal slabs laid across the
top of the uprights.

The second principle of megalithic architecture was the use of more or
less coursed masonry set without mortar, each block lying on its side
and not on its edge. It is quite possible that this principle is less
ancient in origin than that of the orthostatic slab, for it usually
occurs in structures of a more advanced type. Thus in simple and
primitive types of building such as the dolmen it is most rare to find
dry masonry, but in the advanced corridor-tombs of Ireland, the Giants'
Graves and _nuraghi_ of Sardinia, and in the 'temples' of Malta this
technique is largely used, often in combination with the upright slab
system. Indeed, this combination is quite typical of the best megalithic