15.3.3.1 Commas
  You can place a comma anywhere in a number field to the
  left of the decimal point or to the right of the field's first
  character. A comma cannot start a format field. BASIC
  prints a comma to the left of every third digit from the deci-
  mal point. If there are fewer than four digits to the left of the
  decimal point, BASIC omits the comma.
  PRINT USING "##,###",10000
  PRINT USING "##,###",759
  PRINT USING "$$#,###.##",25694.3
  PRINT USING "**#,###",7259
  PRINT USING "####,#.##",25239
  END

  Output
    10,000
        759
    $25,694.30
    **7,259
    25,239.00