Of course, you know, what are the seven basic sounds: do, re, mi, FA, Sol, La, si. However, these names appeared later than the letter.
Letters began to be applied in Europe to represent sounds from the X century. While the lowest used then in the singing sounds - La - became known as the first letter of the Latin alphabet. In ascending order of sounds are listed as follows: a (a), (b-flat), C (), D (re), E (mi), F (f), G (Sol).
When there was the need to sing not only in b-flat, but higher sound, C, called the next letter is N. To denote alterering sounds (if you forgot what alteration, read about it) began to apply the syllables is the increase of a semitone) and es (fall).
Thus, the sound sharp in a letter designation cis and d flat - des; note g-double-sharp - gisis and Sol-double-flat - geses. With the exception of e-flat, a-flat and b-flat. They, respectively, are denoted as es, as, and b.
With letter designations can be found listening to the concert; often the executable announce: Sonata cis-moll, Polonaise As-dur, Mazurka a-moll... Dur here means major, minor - minor. Thus, cis-moll - in C-sharp minor, As-dur - La-flat major.
When a letter designation a major key is usually written with capital letters, and minor - with a lowercase.