A third of the circle is one hundred and twenty degrees, and three signs of the same element stand at that distance from one another. Fire trines fire, earth trines earth, air trines air, water trines water. The trine and the elemental scheme are therefore not two separate doctrines that happen to agree; they are one arrangement of the zodiac seen from two directions. This is worth stating because it explains why the trine was never controversial. It falls out of the structure rather than being imposed on it.
Ancient sources rank the trine as the most favourable of the configurations, and the reason given is geometric rather than sentimental: it joins signs that share a nature, so the planets involved are not asked to reconcile anything. The opposition joins signs of contrary nature, the square joins signs that share a mode but not an element, and the trine joins signs that agree throughout. Hellenistic writers use the term for it that also means agreement.
The modern addition is a caution rather than a technique. Nineteenth and twentieth century writers began to treat the trine as a place where nothing is demanded and therefore nothing is developed, an ease that can become inertia. That reading has no ancient warrant and should not be presented as though it did, but it is now common enough that a reader encountering a chart discussion of trines will meet it. Orbs for the trine are generally set wide, on both the traditional planetary system and the modern aspect-based one, which makes it one of the aspects least affected by which convention a practitioner follows.