Compound words are words that add together to make a new word, like the word "side" and "walk", to make "sidewalk". These words often develop over time. They can start as two words joined with a hyphen, and then become a compound word. There are three types of compound words; closed, hyphenated and open. A closed compound word is joined into one word like "keyboard" or "makeup". A hyphenated word is two words joined with a hyphen like "mother-in-law". Open compound words are words which are used together like "police station", or "fire engine".