Middle English phonology is necessarily somewhat speculative, since it is preserved only as a written language. Nevertheless, there is a very large text corpus of Middle English. The dialects of Middle English vary greatly over both time and place, and in contrast with Old English and Modern English, spelling was usually phonetic rather than conventional. Words were generally spelled according to how they sounded to the person writing a text, rather than according to a formalised system that might not accurately represent the way the writer's dialect was pronounced, as Modern English is today.