You add -th to numbers written in figures and ending in 4, 5, 6, 7, 8, 9, 10, 11, 12, or 13 in order to form ordinal numbers. These numbers are pronounced as if they were written as words. For example, 7th is pronounced the same as `seventh', and 5th is pronounced the same as `fifth'.
...Thursday, 10th May, 1990.
...between Broadway and 6th Avenue.
...the 25th amendment to the American constitution.
English dictionary. 2008.