Collocation is the propensity for some words to appear together in natural language, and is one of the hardest things for EFL students to learn. There is often no other reason, for example, for native speakers to usually say 'tall buildings' and 'high mountains' rather than 'high buildings' and 'tall mountains', beyond observing that the former are more highly collocated. Collocation is useful, because familiar groups of words can be comprehended more quickly and make communication more efficient.
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