An atom in Strawberry Prolog is either an arbitrary series of letters, digits, and/or underscores, that begins with a lowercase letter, or a series of characters from the set

# $ * + - . / : < = > ? @ ^ ~ \ ,

or the special atom [] , or a series of arbitrary characters in single quotes ('). Within single quotes a single quote is written double, e. g. 'can''t do'.

Note, that the uppercase letter or underscore that the name of a variable begins with is all that makes the difference between a variable and an atom.