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 when combined, they constitute the most flexible and expressive instrumental chorus—the natural nucleus of the orchestra. Though notable achievements with both classes were delayed till later, the 17th century is remembered for its persistent endeavors to perfect them and to discover styles appropriate to them.

101. The Keyboard.—The genesis of the modern keyboard cannot be fully traced. Its essential mechanical principle (that of the lever) has always been obvious, but its special adaptation to the hand and to the production of tone must have been gradually worked out. By the 16th century the keyboard as now known had become well established, though some niceties of measurement and disposition were not fully settled.

In modern usage 13 (white) keys occupy the lateral space of one foot, so that an octave is about 6-½ in. wide. The white keys are 7/8 in. wide on top, and the black keys 3/5 in., the latter being about 2 in. behind the former and about ½ in. higher. The dip of all is usually about 3/8 in. All these details are the result of centuries of experiment (see sec. 32).

The peculiar disposition of the longer and shorter keys—the latter interspersed among the former in twos and threes alternately—is really arbitrary, but it clearly records a stage in the evolution of musical theory to which the staff-notation is also accommodated. Although both keyboard and notation are mediæval, they are so imbedded in musical praxis and terminology that apparently no more scientific substitutes are likely to come into general use.

So long as the mediæval modes were used in their purity, only what are now white keys were required. The need of the chromatic semitones probably arose both from the desire to transpose diatonic melodies and from the growing use of musica ficta, with its virtual acceptance of what is now called modulation. Possibly the semitones came in gradually in some such order as this—Bb, F#, C#, Eb, G#. Their tuning offered a problem not well solved till the 18th century. To avoid harshness, alternative semitones were sometimes provided, as both G# and Ab between G and A. The diatonic keys were often colored black and the semitones white—the reverse of present-day practice. Semitones were at first not introduced throughout, but were confined to the middle octaves.

The length or compass of the keyboard has varied greatly. In early organs simply a single octave or an octave and a half was not uncommon, since only this limited range was needed to carry Plain-Song melodies. But as the notion of harmony and