What is a Compound Preposition?

The prepositions formed by prefixing/suffixing a simple preposition to a noun, an adjective or an adverb are called Compound Prepositions.

Examples:

1. There is a long bridge across the river.
2. She quit the job against my advice.
3. His strange behaviour is beyond my comprehension
4. There can be no life without water.
5. Who were you talking about that day?

The highlighted words in the above sentences (across, against, beyond, without, and about) are, by definition, compound prepositions.