chickadee » srfi-14 » char-set:letter

char-set:letterconstant

There are 52 ASCII letters

 abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ

For Unicode, a character is a letter if it has a Alphabetic property.

Some Unicode letters:

05D0HEBREW LETTER ALEF
3042HIRAGANA LETTER A
0345COMBINING GREEK YPOGEGRAMMENI