Prefixes are key morphemes in English vocabulary that begin words. The English prefix *ob-* usually means "against, in the way" or "towards." An example of *ob-* when it means "towards" is the word [*ob*]ligation, or a tying of some duty "towards" you; in the case of *ob-* meaning "against" or "in the way," the word [*ob*]stacle" refers to that which stands "against" you, "in the way" of proceeding from point A to point B.