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You can have some experience of the notion of octave for yourself. If you go to a piano keyboard you will notice that the names of the pitches associated with each key repeats: C-C#-D-D#-E-F-F#-G-G#-A-A#-B-C-C# . . . and so on. Here, the C repeats, as does the C#, and so on. If you go to a piano and play all the keys named"C", you will notice that each tone sounds alike. Then try it yourself: click on any key and try to find the octave above it.
To get some feeling for this phenomenon, click on the "Start PlayList 1" button in the following example. You will notice how each of the "C"'s played sound, in some way, alike.
This alikeness has to do with the fact that each tone is separated by an octave. Numerically, an octave is a doubling in frequency. In the above example, the lowest tone (C3) has a frequency of 130 Hz. The second tone (C4) in the sequence has a frequency of 260 Hz, the third (C5) 520 Hz, and so on. Notice that as the frequency increases, so too does the number of cps (Hz) constituting the octave.