"rent-a-cop", "mother-of-pearl" and "hide-and-seek".
*The solid or closed form in which two usually moderately short words appear together as one. Solid compounds most likely consist of short (monosyllabic) units that often have been established in the language for a long time. Examples are "housewife", "lawsuit", "wallpaper", "basketball".

Usage in the US and in the UK differs and often depends on the individual choice of the writer rather than on a hard-and-fast rule; therefore, spaced, hyphenated,