Tri-

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Tri- or tria- is a numerial prefix meaning 3 that can be either Greek or Latin; though occasionally tre- and ter- also exist for Latin. Examples are:

A trioxide has 3 oxygen atoms
A tricycle is a children's vehicle with 3 wheels
A trilogy is a series of 3 plays or books
The Triassic Period had 3 layers of rock.
A triad has 3 musical notes.

"Tri-" is now a productive prefix in English, even for words that are not of immediate Latin or Greek origin:

A trimonthly event occurs every three months.
A tri-captain (U.S. only?) is one of three equal captains of a sports team.

Examples of the rare English "ter-" and "tre-" are:

"tercentenary", a 300th anniversary.
"ternary" (rather than trinary), the analogue of "binary"
"tertiary"
"treble"
"trefoil"

The Sanskrit cognate "tri-" has been used as a prefix for unknown elements indicating that it is three rows below the nearest known element in the periodic table.

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