We have 12 notes in music. If we play each next available key on the piano we are playing the chromatic scale. A half step means to go up one semi-tone ( the next note whether black or white key) and a whole step means to go up two semi-tones. 

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These are all the 12 notes in music:

A , A#, B , C , C# , D , D# , E , F , F# , G , G#

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If we play it coming back doing the notes are:

A , Ab , G , Gb , F , E , Eb , D , Db , C , B , Bb 

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# (sharp) = raise the note

b (flat) = lower the note