Obligatio is defined in Justinian’s Institutes as a tie of law, a legal relationship between two persons whereby one is constrained by the other to do or refrain from doing something. It brings together relationships arising out of contract or delict, though the Digest shows it used more generally wherever a personal bond was created. Its roots lie in the verb ligare, to bind; but although Roman lawyers preferred the use of verbs over abstract nouns, here the noun form is almost as common as the verb. As a noun obligatio describes either the active or the passive aspect of the relationship or the relationship itself, allowing flexibility in legal thinking. Originally, obligatio may have been related to actio, so that only enforceable relationships were included within the word, but by classical law it applied to any relationship with legal consequences, whether or not the relationship was enforceable.