Fraction numbers describes how many parts of an entire there are. Below the fraction bar is the whole and above the parts of this whole that are actually there. 3/5 means that from the five parts three are actually there.
What is above the bar is called the contatore / numerator and what is under the bar is called the denominatore / denominator. The numerator is no problem, it is just a cardinal number. Except intero and mezzo the denominators doesn't have an own form neither or in other words the ordinal numbers are used.
summary
1/1 = un intero
1/10 = un decimo
1/2 = un mezzo
1/12 = un dodicesimo
1/3 = un terzo
1/13 = un tredicesimo
1/4 = un quarto
1/14 = un quattordicesimo
1/5 = un quinto
1/15 = un quindicesimo
1/6 = un sesto
1/16 = un sedicesimo
1/7 = un settimo
1/17 = un diciasettesimo
1/8 = un ottavo
1/18 = un diciottesimo
1/9 = un nono
1/19 = un dicinuovesimo
If the numeratore is bigger than one the denominatore obviously must be in plural.
