58. Joined together!

In English Language these are a pair or grouping of words that is used together as an idiomatic expression, usually conjoined by the words and or or. Expressions like `to and fro', `safe and sound', `take it or leave it', and `rhyme or reason' are some of the examples of the grouping. The two words in these expressions are inseparable; their order is fixed and they have only one meaning. What are such pairs called?