An ordinal number tells you the position or order of something in a sequence: first, second, third, fourth, and so on. While cardinal numbers (1, 2, 3, 4) tell you how many things you have, ordinal numbers tell you where something ranks or appears in line.

Think about a race: if 20 students compete, cardinal numbers count all 20 runners, but ordinal numbers identify who finished first, who finished second, and who came in twentieth. When your teacher calls on the third person in the row, or you live on the ninth floor of a building, or you're reading the twelfth chapter of a book, you're using ordinal numbers to describe position.

Notice that ordinal numbers usually end in “th” (fourth, fifth, sixth), with three common exceptions: first, second, and third. You can write them as words or use numbers with letter endings: 1st, 2nd, 3rd, 4th, 5th, and so on.

Ordinal numbers matter whenever order or sequence is important: finishing places in competitions, floors in buildings, steps in instructions, or dates in months (June 14th is the fourteenth day of June). They help us organize information and describe relationships that involve ranking or arrangement rather than just counting.