common

The root common comes from Latin commūnis, built from com- ('together') + mūnis ('bound to a duty or service'). So at its core, commūnis did not just mean 'shared' — it meant 'sharing the burden,' the way fellow citizens or villagers all pitch in. From 'jointly obliged' it slid naturally to 'belonging to all,' then to 'public,' and finally to plain 'ordinary.'

That slide is the whole story of the English family. common itself runs the full ladder: 'shared by a group' (a common goal, common ground) → 'belonging to the public' (the common good) → 'found everywhere' (a common bird) → 'ordinary, nothing special' (a common name). One word carries three centuries of drift, from the noble idea of shared duty down to the slightly dismissive 'common.'

The prefixes and suffixes then spin off the rest:
- commonly / uncommon / uncommonly ride the 'found everywhere' sense: commonly = usually; uncommon = rare; uncommonly = to an unusual degree.
- commonplace comes from rhetoric: a locus communis (Latin) was a 'common place' — a stock argument every speaker kept ready. Because such lines were shared by all, a 'commonplace' became a worn-out cliché, and the adjective drifted to 'ordinary, unremarkable.'
- commons is the 'belonging to all' sense made concrete: shared grazing land (the village commons), then the ordinary people who used it (the commons = commoners), and in Britain the House of Commons — the chamber of the common people.
- commonwealth = common + weal ('well-being'): the shared welfare. It named the whole body politic organized for the common good — hence republics, the US states styled 'Commonwealths,' and the Commonwealth of Nations.
- commune keeps the rawest sense of commūnis: to hold things in common. As a verb, to commune is to share thoughts or feelings so intimately you have something in common; as a noun, a commune is a community that pools property and work.

The same commūnis also seeded words we don't spell with -common-: community and communicate (to make something common, i.e. to share it), and communism (the doctrine of common ownership). All of these trace back through the mun- element (Latin mūnus / mūnia = 'duty, service, gift'), the same root behind municipal (a town that takes on its own civic duties) and immune (literally 'exempt from duty,' im- 'not' + mūnis). So the surprising cousin of 'common' is 'immune' — one means 'sharing the duty,' the other 'let off the duty.'

The pattern to hold onto: com- (together) + mun- (duty/service) = jointly obligated → shared → public → ordinary. Every member of the family sits somewhere on that ladder.