The _monochord_ was mounted with a single string stretched over two bridges which were fixed on an oblong box. The string could be tightened or slackened by means of a turning screw inserted into one end of the box. The intervals of the scale were marked on the side, and were regulated by a sort of movable bridge placed beneath the string when required. As might be expected, the _monochord_ was chiefly used by theorists; for any musical performance it was but little suitable. About a thousand years ago when this monochord was in use the musical scale was diatonic, with the exception of the interval of the seventh, which was chromatic inasmuch as both _b-flat_ and _b-natural_ formed part of the scale. This ought to be borne in mind in examining the representations of musical instruments transmitted to us from that period. As regards the wind instruments popular during the Middle Ages, some were of quaint form as well as of rude construction. [Illustration: FIG. 31.――ORGANISTRUM.] The _chorus_, or _choron_, had either one or two tubes. There were several varieties of this instrument; sometimes it was constructed with a bladder into which the tube is inserted; this kind of _chorus_ resembled the bagpipe; another kind resembled the _pungi_ of the Hindus, mentioned on page 52. The name _chorus_ was also applied to certain stringed instruments. One of these had much the form of the _cithara_, page 84. It appears, however, probable that _chorus_ or _choron_ originally designated a horn (Hebrew, _keren_; Greek, _keras_; Latin, _cornu_). The flutes of the Middle Ages were blown at the end, like the flageolet. Of the _syrinx_ there are extant some illustrations of the ninth and tenth centuries, which exhibit the instrument with a number of tubes tied together, just like the Pandean pipe still in use. In one specimen,[6] from a manuscript of the eleventh century, the tubes were inserted into a bowl-shaped box. This is probably the _frestele_, _fretel_, or _fretian_, which in the twelfth and thirteenth centuries was in favour with the French ménétriers. Some large Anglo-Saxon trumpets may be seen in a manuscript of the eighth century in the British Museum. The largest kind of trumpet was placed on a stand when blown. Of the _oliphant_, or hunting horn, some fine specimens are in the Victoria and Albert Museum collection. The _sackbut_ (Fig. 32), probably made of metal, could be drawn out to alter the pitch of sound. The sackbut of the ninth century had, however, a very different shape to that in use about three centuries ago, and much more resembled the present _trombone_. The name _sackbut_ is supposed to be a corruption of _sambuca_. The French, about the fifteenth century, called it _sacqueboute_ and _saquebutte_. [Illustration: FIG. 32.――SACKBUT.] The most important wind instrument――in fact, the king of all the musical instruments――is the organ.