This article looks at how a comprehensive list of one category of idioms, that of "core idioms", was established. When the criteria to define a core idiom were strictly applied to a dictionary of idioms, the result was that the large number of "idioms" was reduced to a small number of "core idioms". The original list from the first source dictionary was added to by applying the same criteria to other idiom dictionaries, and other sources of idioms. Once the list was complete, a corpus search of the final total of 104 "core idioms" was carried out in the British National Corpus (BNC). The search revealed that none of the 104 core idioms occurs frequently enough to merit inclusion in the 5,000 most frequent words of English.