Input is usually provided by the player in the form of simple sentences such as "get key" or "go east", which are interpreted by a text parser. Parsers may vary in sophistication; the first text adventure parsers could only handle two-word sentences in the form of verb-noun pairs. Later parsers, such as those built on ZIL (Zork Implementation Language), could understand complete sentences. Later parsers could handle increasing levels of complexity parsing sentences such as "open the red box with the green key then go north". This level of complexity is the standard for works of interactive fiction today.