A symbol is one or more characters chosen from the set of all letters (both upper and lower case), digits and the three characters "_.$". On most machines, you can also use $ in symbol names; exceptions are noted in Chapter 30. No symbol may begin with a digit. Case is significant. There is no length limit: all characters are significant. Symbols are delimited by characters not in that set, or by the beginning of a file (since the source program must end with a newline, the end of a file is not a possible symbol delimiter). See Chapter 27.