In practice, a purely-synchronic linguistics is not possible for any period before the invention of the gramophone, as written records always lag behind speech in reflecting linguistic developments. Written records are difficult to date accurately before the development of the modern title page. Often, dating must rely on contextual historical evidence such as inscriptions, or modern technology, such as carbon dating, can be used to ascertain dates of varying accuracy. Also, the work of sociolinguists on linguistic variation has shown synchronic states are not uniform: the speech habits of older and younger speakers differ in ways that point to language change. Synchronic variation is linguistic change in progress.