One of the main challenges for research in the field of Phraseology is to discover how phraseological combinations can be integrated into the grammatical rules they seem to contradict. This paper explains some theoretical concepts which may shed some light on the relation between regular syntax and phraseological fixedness. Firstly, we explain how the concept of grammatical metaphor, applied to phraseology, allows to distinguish phrasemes from free combinations, and also to separate the phrasemic subclasses from one another. Secondly, we analyze the different degrees of fixedness of phrasemes in terms of functional idiomaticity, as a consequence of grammatical metaphor, resulting from the relation between the inner and outer syntax of the phrasemes, which is parallel to their semantic idiomaticity. Thirdly, we focus on some particular cases of this mechanism, such as discontinuous, interlocking and overlapping phrasemes, as well as the so-called open-slot idioms. We conclude that the application of grammatical metaphor to phraseology brings up to surface a parallelism between idiomatic meanings and idiomatic structures, which is an essential property of phrasemes.