When enharmonic notes are combined in larger musical structures, those structures are also enharmonic. Intervals, scales and key signatures may be enharmonic.
An interval, or a set of two notes, can be enharmonic to another interval if the pitches of both intervals are the same. In the example below, the interval of Gb and Db is enharmonic to the interval of F# and C# because Gb is enharmonic to F# and Db is enharmonic to C#.
An interval may be enharmonic if it has a different numeric value. Gb and B is enharmonic to F# and Cb, even though Gb to B is a third and F# to Cb is a fifth.
A scale is enharmonic to another scale if every note in both scales is enharmonic. The following scales, B Major and Cb Major, are enharmonic because each note in a scale is enharmonic to the corresponding note in the other scale. B is enharmonic to Cb, C# is enharmonic to Db, and so on.
Enharmonic Key Signatures
Since scales can be enharmonic, it makes sense that key signatures (which define the notes of a scale) can also be enharmonic. The key of Cb Major is enharmonic to the key of B Major.