1911 Encyclopædia Britannica/Octave
OCTAVE (from Lat. octavus, eighth, octo, eight), a period or series of eight members. In ecclesiastical usage the octave is the eighth day after a particular church festival, the feast day itself and the “octave” being counted. The octave thus always falls on the same day of the week as the festival, and any event occurring during the period is said to be “in the octave.” In music, an octave is the eighth full tone above or below any given note. It is produced by double or half the number of vibrations corresponding to the given note. In the interval between a note and its octave is contained the full scale, the octave of a note forming the starting-point of another scale of similar intervals to the first. The interval between a note and its octave is also called an octave. The name is also applied to an open metal stop in an organ, and to a flute (more usually known as the piccolo) one octave higher in pitch than the regular flute. It is also a term for a “parade” in fencing. The “law of octaves” was a term applied in 1865 to a relationship among the chemical elements enunciated by J. A. R. Newlands.
In literature an octave is a form of verse consisting of eight iambic lines, and complete in itself. From its use by the poets of Sicily, the recognized type of this form is usually called the Sicilian Octave. It is distinguished from a single stanza of ottava rima, in which the rhyme-arrangement is abababcc, by having only two rhymes, arranged abababab. In German literature the octave has been used not infrequently since 1820, when Ruckert published “Sicilianen,” as they are called in German, for the first time. The word octave is also often used to describe the eight opening lines of a sonnet, in which the rhyme-arrangement is abbaabba, or some modification of this, but properly always on two rhymes only.