Il numero romano CDXCI nel sistema decimale vale: 491.
CDXCI in latino si scrive quadringenti nonaginta unus,
mentre in italiano quattrocentonovantuno.
CDLXXII, CDLXXIII, CDLXXIV, CDLXXV, CDLXXVI, CDLXXVII, CDLXXVIII, CDLXXIX, CDLXXX, CDLXXXI, CDLXXXII, CDLXXXIII, CDLXXXIV, CDLXXXV, CDLXXXVI, CDLXXXVII, CDLXXXVIII, CDLXXXIX, CDXC, CDXCII, CDXCIII, CDXCIV, CDXCV, CDXCVI, CDXCVII, CDXCVIII, CDXCIX, D, DI, DII, DIII, DIV, DV, DVI, DVII, DVIII, DIX, DX