What exactly is a megalithic monument? Strictly speaking, it is a building made of very large stones. This definition would, of course, include numbers of buildings of the present day and of the medieval and classical periods, while many of the Egyptian pyramids and temples would at once suggest themselves as excellent examples of this type of building. The archaeologist, however, uses the term in a much more limited sense. He confines it to a series of tombs and buildings constructed in Western Asia, in North Africa, and in certain parts of Europe, towards the end of the neolithic period and during part of the copper and bronze ages which followed it. The structures are usually, though not quite invariably, made of large blocks of unworked or slightly worked stone, and they conform to certain definite types. The best known of these types are as follows: Firstly, the menhir, which is a tall, rough pillar of stone with its base fixed into the earth. Secondly, the trilithon, which consists of a pair of tall stones set at a short distance apart supporting a third stone laid across the top. Thirdly, the dolmen, which is a single slab of stone supported by several others arranged in such a way as to enclose a space or chamber beneath it. Some English writers apply the term cromlech to such a structure, quite incorrectly. Both menhir and dolmen are Breton words, these two types of megalithic monument being particularly frequent in Brittany. Menhir is derived from the Breton _men_, a stone, and _hir_, long; similarly dolmen is from _dol_, a table, and _men_, a stone. Some archaeologists also apply the word dolmen to rectangular chambers roofed with more than one slab. We have carefully avoided this practice, always classing such chambers as corridor-tombs of an elementary type. Fourthly, we have the corridor-tomb (_Ganggrab_), which usually consists of a chamber entered by a gallery or corridor. In cases where the chamber is no wider than, and hence indistinguishable from the corridor, the tomb becomes a long rectangular gallery, and answers to the French _allee couverte_ in the strict sense. Fifthly, we come to the _alignement_, in which a series of menhirs is arranged in open lines on some definite system. We shall find a famous example of this at Morbihan in Brittany. Sixthly, there is the cromlech (from _crom_, curve, and _lec'h_, a stone), which consists of a number of menhirs arranged to enclose a space, circular, elliptical or, in rare cases, rectangular. These are the chief types of megalithic monument, but there are others which, though clearly belonging to the same class of structure, show special forms and are more complicated. They are in many cases developments of one or more of the simple types, and will be treated specially in their proper places. Such monuments are the _nuraghi_ of Sardinia and the 'temples' of Malta and Gozo.