Tertium comparationis (Latin = the third [part] of the comparison) is the quality that two things which are being compared have in common. It is the point of comparison which prompted the author of the comparison in question to liken someone or something to someone or something else in the first place.

If a comparison visualizes an action, state, quality, object, or a person by means of a parallel which is drawn to a different entity, the two things which are being compared must necessarily not be identical. However, they must possess at least one quality in common. This common quality has traditionally been referred to as tertium comparationis.

The most common devices used to achieve this are metaphors and similes, especially, but by no means exclusively, in poetic language. In many cases one aspect of the comparison is implied rather than made explicit.

Examples

• Necessity is the mother of invention. (English proverb)

Objects of comparison: relationship between mother and child, relationship between necessity and invention

Tertium comparationis: source, where something derives from

• Woman is the nigger of the world. (John Lennon)

Objects of comparison: treatment of blacks by US culture, treatment of women by the world culture

Tertium comparationis: inhumane treatment, subjugation, discrimination

• Goodbye, England's rose. (Elton John on the death of Diana, Princess of Wales)

Objects of comparison: rose; Diana, Princess of Wales

Tertium comparationis: beauty

If they [our two souls] be two, they are two so

As stiff twin compasses are two;

Thy soul, the fixed foot, makes no show

To move, but doth, if th'other do.

(John Donne: "A Valediction: Forbidding Mourning") (Read the whole poem (http://eir.library.utoronto.ca/rpo/display/poem682.html).)

Objects of comparison: two souls; twin compasses

Tertium comparationis: a non-physical link between separate objects that causes action in one to result in action to the other.