The rules determining the forms of nouns and adjectives after numerals and the associated initial-consonant mutations in Middle Welsh are often puzzling to Modern Welsh readers of the language. This article sketches the rules and provides a synchronic account of them. It is shown that they are based on a coherent system where number (singular, dual, numerative, plural) is central. The linguistic changes that have occurred since then are documented in detail and dated using textual evidence. It is argued that these changes can best be understood as steps along a pathway towards a new, equally coherent system where all numeral phrases are treated as singular, and gender rather than number determines both form and mutations.