What is a combining vowel and when is it used in medical terms?

A combining vowel, usually the letter O, is placed between a root word and a suffix or between two root words to make the term easier to pronounce. The combining vowel is used when the suffix begins with a consonant, such as gastroscopy where the O connects gastr to scopy. It is dropped when the suffix begins with a vowel, such as gastritis where gastr connects directly to itis. The root word plus the combining vowel is called the combining form. For example, the combining form of cardi is cardio, as in cardiovascular. This rule applies consistently across medical terminology.