Core Location: Theory and Mapping
We start with our musical ruler...
There are 12 tones. They are 1 half step [1 fret] apart (up or down to the next). Say it. Know it.
The sharp [♯] raises a natural one half step and the flat [♭] lowers a natural one half step. These tones have 2 names. Theoretically, every tone has infinite names (B one million sharp is?), but we can lose friends talking like this. We keep it simple...as simple as possible. Double flats and sharps do exist. And, B is C♭, C is B♯, E is F♭, and F is E♯ (these are used).
There are 2 more naturals than sharps/flats, therefore, there are two place that naturals 'touch'.
These are naturally occurring half steps between B/C & E/F.