The octave is a perfect interval in which the upper note vibrates at exactly twice the frequency of the lower (2:1). Because of this physical fact, different octaves are heard as the same pitch quality.
A piano covers 7 octaves, a violin about 4, the human voice 2–3 (rare talents like Mariah Carey reach 5). Parallel octaves in a melody — two parts moving identically a full octave apart — are a classic device of reinforcement, heard in Verdi overtures and Mahler symphonies.
The phenomenon by which pitches an octave apart are heard as “the same” is called octave equivalence: this is why every C on the keyboard shares one name. The octave is divided into 12 equal semitones in equal temperament — a compromise system that allowed free modulation between keys. The registers of instruments and voices are measured precisely in octaves from a notional centre.
