Summary: Prefixes are key morphemes in English vocabulary that begin words. A fair number of English vocabulary words contain the prefix *dia-*, which means "across." Examples using this prefix include [*dia*]logue, [*dia*]gonal, and [*dia*]betes. An easy way to remember that the prefix *dia-* means "across" is through the word [*dia*]meter, for the [*dia*]meter of a circle is the measurement "across" it.
