sol

The root sol is a coincidence — two completely unrelated Latin words that ended up looking identical in English.

The first is sōlus, meaning 'alone, single, by oneself.' This is the 'one and only' branch. A Roman who acted sōlus acted with no one beside him. From this single idea English built a whole family:

- sole / solely — the only one, and only: the sole survivor, solely responsible.
- solo / soloist — a performance by one person alone, borrowed through Italian music.
- solitary / solitude — being alone, the state of aloneness.
- desolate — de- (completely) + sōlus = 'made utterly alone.' A place emptied of all people is desolate; a person stripped of all comfort feels desolate. The leap from 'alone' to 'ruined, bleak' is the emotional weight of total abandonment.
- soliloquy — sōlus (alone) + loquī (to speak) = 'speaking alone.' On stage, a character delivers a soliloquy when they voice their thoughts with no one else listening — Hamlet's 'To be or not to be.'

The second word is sōl, simply 'the sun.' This is the sky branch, and it stays concrete:

- solar — of the sun: solar energy, the solar system.
- parasol — para- (guard against) + sōl = a little shade you carry to guard against the sun, the sunny cousin of the umbrella (which guards against rain).
- insolate — to expose to the sun's rays; solarize — to overexpose to sunlight.
- extrasolar / circumsolar — outside / around the sun.
- turnsole — turn + sōl, a plant that turns to follow the sun.

How do you tell the branches apart? Ask what the word is really about. If it is about being one or alone — sole, solitude, solo — it is sōlus. If it is about the sun in the sky — solar, parasol — it is sōl. The two never actually mean the same thing, so context decides instantly.

A warning about look-alikes: several common words wear sol- but belong to neither branch. solid and consolidate come from solidus ('firm, whole') — about hardness, not aloneness or the sun. solemn and its family come from sollemnis ('ceremonial'). console ('to comfort') comes from sōlārī ('to comfort'). obsolete comes from obsolēscere ('to fall out of use'). And isolated, despite meaning 'alone,' actually comes from Latin insula ('island') by way of Italian — the resemblance to sōlus is a pure accident of meaning.