xxxi
xxxi is the Roman numeral form of thirty-one, or 31. You may see it marking a page, chapter, outline section, or traditional event name, where Roman numerals give a formal sequence instead of an everyday number. But should you write xxxi, XXXI, 31, or thirty-one? This lesson looks at how xxxi works like an adjective-like numeral, why uppercase is usually preferred, and when the modern forms are clearer. Watch the full lesson to learn how to recognize and use xxxi naturally.