  There are several notions of the 'dual' of a word/tile substitution. We show
that the most common ones are equivalent for substitutions in dimension one,
where we restrict ourselves to the case of two letters/tiles. Furthermore, we
obtain necessary and sufficient arithmetic conditions for substitutions being
selfdual in this case. Since many connections between the different notions of
word/tile substitution are discussed, this paper may also serve as a survey
paper on this topic.
