Il numero romano CDIX nel sistema decimale vale: 409.
CDIX in latino si scrive quadringenti novem,
mentre in italiano quattrocentonove.
CCCXC, CCCXCI, CCCXCII, CCCXCIII, CCCXCIV, CCCXCV, CCCXCVI, CCCXCVII, CCCXCVIII, CCCXCIX, CD, CDI, CDII, CDIII, CDIV, CDV, CDVI, CDVII, CDVIII, CDX, CDXI, CDXII, CDXIII, CDXIV, CDXV, CDXVI, CDXVII, CDXVIII, CDXIX, CDXX, CDXXI, CDXXII, CDXXIII, CDXXIV, CDXXV, CDXXVI, CDXXVII, CDXXVIII