Il numero romano CDXXI nel sistema decimale vale: 421.
CDXXI in latino si scrive quadringenti viginti unus,
mentre in italiano quattrocentoventuno.
CDII, CDIII, CDIV, CDV, CDVI, CDVII, CDVIII, CDIX, CDX, CDXI, CDXII, CDXIII, CDXIV, CDXV, CDXVI, CDXVII, CDXVIII, CDXIX, CDXX, CDXXII, CDXXIII, CDXXIV, CDXXV, CDXXVI, CDXXVII, CDXXVIII, CDXXIX, CDXXX, CDXXXI, CDXXXII, CDXXXIII, CDXXXIV, CDXXXV, CDXXXVI, CDXXXVII, CDXXXVIII, CDXXXIX, CDXL