to generate a string in the language, one begins with a string consisting of only a single "start symbol", and then successively applies the rules (any number of times, in any order) to rewrite this string. this stops when we obtain a string containing only terminals. the language consists of all the strings that can be generated in this manner. any particular sequence of legal choices taken during this rewriting process yields one particular string in the language. if there are multiple different ways of generating this single string, then the grammar is said to be ambiguous.