metro

This root means mother, and it reaches English by two roads that started from the same Proto-Indo-European word, *méh₂tēr.

The Greek road gave mētēr (mother), which combines with pólis (city) to form metropolis — literally the 'mother city.' In ancient Greece, when a city grew too crowded it sent settlers off to found colonies; the original city was their mother, their mētrópolis. The colonies are long gone, but the word survives meaning a great central city, and its adjective metropolitan now simply means 'of a big city and its surrounding region.'

The Latin road gave māter (mother) and its stem mātr-, which feeds a different cluster:

- mātr- + -ity → maternity (the state of being a mother; everything around pregnancy and birth)
- mātr- + -mony (state) → matrimony (the state of marriage — historically the institution that made a woman a mother and head of a household)
- mātr- + arch (rule) → matriarchy (rule by the mother), the mirror image of patriarchy

One member needs an honest warning. matriculate ('to enroll at a university') looks like it should be about mothers, but it actually comes from Latin mātrīcula, a 'little register' or list — itself a diminutive of mātrīx, which meant both 'womb' and 'public register.' The womb was imagined as the source-list of life, so a register became a mātrīx, and a small one a mātrīcula; to matriculate is to get your name written into the official list. The 'mother' link is real but buried under two layers of metaphor.

The pattern: when you see metr-/matr- meaning mother, look for the idea of a source or origin — a mother city, the state of motherhood, the founding institution, the rule of the mother. And do not confuse this metr- with the unrelated metr- in metric and thermometer, which comes from Greek métron, 'measure.'