Except where characters are involved in numeric or string constants, the extent of the token containing a particular character depends on to which class the character belongs; unlike TeX, Metafont has fixed character classes. The characters codice_12, codice_13, codice_14, and codice_15 are "loners" and only form single character tokens. For the character classes codice_16, codice_17 (single quotes), codice_18, codice_19, codice_20, codice_21, codice_22, codice_23, codice_24, codice_25, and codice_26, as well as the class of underscore together with upper and lower case A–Z, the token consists of the longest consecutive sequence of characters from the same class. Whitespace characters don't contribute tokens. codice_27 starts a comment lasting until end of line.