S' clitics are a subtype of SPECIAL CLITICS (clitics not partaking of the distribution of corresponding full forms) which function as constituents of S'-i.e. as modifiers of S. Special clitics marking mood, tense, and aspect are typical S' clitics; and those marking subject pronouns are typical examples of S (rather than S') clitics. - Zwicky (2004), a pag.364