Chords by notes
There are twelve different pitches or notes in music. In chord names, the root note is always written out, and therefor chords are often referred to a C chord, a D chord and so on.
The letters: C, D, E, F, G, A and B
So, in music we are using the letters C, D, E, F, G, A and B. These are names of notes or pitches, as well as name of chords. It's not the same thing though, a C note is just a note, whereas C chord includes a couple of notes but with C as the root note.
Sharps (#) and flats (b)
In chord names and in many other circumstances the flats and sharps are written in the symbols # and b, respectively. The flat means that a tone is flattened and the sharp means that a tone is raised.
So, in other words, a Db is a D that is flattened one semi-step (a whole step had made it to a C). And therefore, a D# is a D that is raised one semi-step.
Something that is often confusing for many is that b and # can occur in different chord names, but referring to identical chords. D# and Eb is actually the same tone or chord (if read as a chord symbol). The reason that to symbols are used for the same thing is that the musical context, in which case the key, change. For example, in the key of B major are D# used, whereas Eb is used for the same note in the key of Ab major.
If you're still confused, just remember that D# and Eb is, practically speaking, the same thing. These are all the cases of sharps and flats being the "same thing".
- C# and Db
- D# and Eb
- F# and Gb
- G# and Ab
- A# and Bb