The grammatical description in the earliest books was somewhat vague, so a consensus on usage (influenced by Zamenhof's answers to some questions) developed over time within boundaries set by the initial outline (Auld 1988). Even before the Declaration of Boulogne, the language was remarkably stable; only one set of lexical changes were made in the first year after publication, namely changing "when", "then", "never", "sometimes", "always" from "kian", "tian", "nenian", "ian", "ĉian" to "kiam", "tiam", "neniam" etc., to avoid confusion with the accusative forms of "kia" "what sort of", "tia" "that sort of", etc. Thus Esperanto achieved a stability of structure and grammar similar to that which natural languages enjoy by virtue of their native speakers and established bodies of literature. One could learn Esperanto without having it move from underfoot. Changes could and did occur in the language, but only by acquiring widespread popular support; there was no central authority making arbitrary changes, as happened with Volapük and some other languages.