It is argued that multi-word lexemes, existing for all classes of single-word lexemes, are an important and growing extension well deserving more study. A notable part of this is to be found in idioms, too, including grammatical idioms, whose prominent class, that of multi-word idiomatic prepositions is studied here both in English and Czech. In these, a prominent part of prepositions is formed by rules, based on specific structures, and might then be viewed as paradigmatic. Data for this research are drawn from British and Czech National Corpus. A formal classification of this type of idioms is offered that is amply illustrated by corpus examples where a particular stress is laid on their place in their system of formation and their frequencies.