Oct- is eight. Let’s take numbers from 1 to 7 and repeat them: 1-2-3-4-5-6-7-1-2-3-4-5-6-7… The distance between the two 1s is eight notes apart; it’s an octave apart (music pro tip: you’ll have to start counting from the first number, not from the second). The distance between the two 5s is an octave apart. Now, if 1 is Do, 2 is Re, 3 is Mi and so on, two consecutive ‘La’ notes will be an octave apart. And if we take their frequencies, the high ‘La’ will have twice the frequency of the low ‘La’ (example: low La: 440 Hz, high La: 880 Hz).