-th¹

Also ‑eth.

Forming ordinal and fractional numbers.

Old English ‑(o)tha.

The ending is added to numbers from four onwards to make ordinal numbers that define a thing's position in a series: fourth, fifth, eighth, twenty-ninth, hundredth. When the number ends in y, that letter changes to i and the ending becomes ‑eth: twentieth, fiftieth, ninetieth. The same form is used in fractions: two fifths, three eighths. See also ‑teen (for ‑teenth).