Every major scale contains one note with each name (ABCDEFG). For example, Bb major is Bb, C, D, Eb, F, G, A. If you start with C major (no sharps), and ascend

Every major scale contains one note with each name (ABCDEFG). For example, Bb major is Bb, C, D, Eb, F, G, A. If you start with C major (no sharps), and ascend in 5ths (G, D, A, etc.) then you add one sharp to the previous major scale to get the major scale starting on the new note. The added sharp each time is the 7th of the scale. So G major just has F#, D major adds also C#, A major adds also G# etc. If you start with C major (no flats), and descend in 5ths (equivalently: ascend in 4ths) (F, Bb, Eb, etc.) then you add one flat to the previous major scale to get the major scale starting on the new note. The added flat each time is the 4th of the scale. So F major just has Bb, Bb major adds also Eb, Eb major adds also Ab etc. Key signatures put the sharps or flats in this order. The two meet in the middle: F# major: F#, G#, A#, B, C#, D#, E# (key with 6 sharps) Gb major: Gb, Ab, Bb, Cb, Db, Eb, F (key with 6 flats) I.e. on the piano F# and Gb are the same key, G# and Ab are the same key etc.