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More facts about the half step:

All letter notes have a sharp, with the exception of B and E. A sharp(#) is the same note as the next letter's flat(b).
There are 12 half steps in an octave.
Each octave up doubles the sound's frequency, so the ratio of the frequency of one half step to the next is one over the twelfth root of two, or approximately 1:1.05946.
(middle C is approx. 261.62 Hz, the A below it is exactly 220 Hz)