A BASIC identifier can consist of from 1  to  31  characters.   The  first
  character of an unquoted string must be an alphabetic character (A through
  Z).  The remaining characters, if  present,  can  be  any  combination  of
  letters,  digits  (0  through  9),  dollar  signs  ($),  periods  (.),  or
  underscores (_).  Embedded blanks are not permitted.

  Internal, implicitly, or explicitly, declared variables can use  the  last
  character  of  the  name  to  indicate  the  data type - a dollar sign ($)
  indicates a string variable, while a percent sign (%) indicates an integer
  variable.   If  the  last  character  of  an internal, implicitly declared
  variable is neither a dollar sign ($) nor a percent  sign  (%),  then  the
  name indicates a variable of the default type.

  A  quoted  name  can  consist  of  any  combination  of  printable   ASCII
  characters.  An identifier cannot be a reserved BASIC keyword.