The texture of notes making up a modern European musical composition is emphasized at successive intervals in general equal, the attack of the separate notes marking out either these moments of intensification or divisions or subdivisions of them, dual or triple. This period of intensification is called a bar, its dual or triple division the beat of the music. Other characteristics in the flow of sound composing a piece of music result in a larger periodic structure; these may be, apart from actual interruptions of continuity, the tendency of musical movement to repeat itself, or to delay upon a long held note or chord, or to change completely in character. A portion of musical texture outlined in this way is called a phrase, subject or theme. The compass of these larger periodicities in composition exhibits a great degree of uniformity, two, four, eight or sixteen bars being their commonest duration. This structure of beats bars and phrases or (larger) periods is commonly explained extra-musically by the bodily conditions of musical and especially vocal, performance.
The fact of accent is related to the pulse beat, to whose rate it in some cases approximates, and that of periods to the breathing, whose interval approximates to that of a four-barperiod. The equalities that subsist between successive larger periods have also been referred to accompanying symmetrical movements of dancing.
An explanation which these facts themselves demand may in its application to the auditory sense be formulated as follows: In the case of any sequence of sound which has begun to recur the ear forms the anticipation of the further elements at the same intervals at which they originally followed one another. The application of this principle to bars of music would lead to their combination into periods according to the powers of the number two.
The charm which these complete units of musical texture called periods or phrases may possess, is one of the most remarkable facts of music. Analogies from other departments of art suggest that their unity itself may be a secret of their beauty; a melody or a complete harmonic sequence impresses us as a growth from a germ of musical form.
In the next lecture we shall close our discussion of musical structure by considering some of the main facts of the simultaneous combination of notes in works of music.