The ordinal form of the number twenty-eight.

The person or thing in the twenty-eighth position. / One of twenty-eight equal parts of a whole.

The ordinal form of the number twenty-one.

The person or thing in the twenty-first position. / One of twenty-one equal parts of a whole, usually written ¹⁄₂₁. / (informal) A party to celebrate a twenty-first birthday.

The ordinal form of the number twenty-nine.

The person or thing in the twenty-ninth position. / Any of twenty-nine equal parts of a whole.

The start of the twenty-first hour of the day on the 24-hour clock, i.e. 20:00.

(idiomatic) very fast, fluently

In the twenty-first place; twenty-first in a row.

A mountain bike with wheels of approximately 29 inches in diameter.