Welsh language
Welsh has mutations, a sound (in speech) or a letter (in writing) changing at the start of a word. Example are the Welsh words "gwneud", which in English means "to do"; and "dod", which means "to come"; and "dewch i mewn", which means "come in". The word sometimes changes from "gwneud" to "wneud" and from "dod" to "ddod". The sounds (in speech) or letters (in writing) changes also occur within and at the end of words, but the simplified classification found in ordinary books does not mention that.