Welsh has mutations. Mutations are when a sound (in speech) or a letter (in writing) changes at the start of a word. An example is the Welsh word ""gwneud"", which in English means "to do", and ""dod"", which means "to come" "dewch i mewn" which means "come in". Sometimes the word changes from ""gwneud"" to ""wneud"", and from ""dod"" to ""ddod"". These sounds (in speech) or letters (in writing) changes also occur within, and at the end, of words, although the simplified classification found in ordinary books does not mention this.