Mexico City — CDMX, and until 2016 the Distrito Federal — holds around 9.2 million people in the city proper and something over 22 million in the metropolitan area, which makes it the largest city in North America.
It sits in a closed basin ringed by mountains at 2,240 metres, on the drained bed of Lake Texcoco. Almost everything distinctive about it follows from those two facts. See The Lake Under the City.
The shape of it
The Centro Histórico is the colonial core, laid out on the ceremonial precinct of Tenochtitlan, with the Zócalo at its centre.
Paseo de la Reforma runs diagonally south-west from there — built by Maximilian in the 1860s so he could see from the castle to the city — through Juárez and past the financial towers to Chapultepec.
Roma and Condesa sit just south of Reforma, and Polanco just north of Chapultepec.
Ten kilometres further south are the absorbed colonial villages of Coyoacán and San Ángel, the UNAM campus, and beyond them Xochimilco and its canals.
The city is divided into sixteen alcaldías — boroughs — of which Cuauhtémoc holds most of what a visitor wants.
Addresses
Written as street, number, then colonia — the neighbourhood — which is essential, because street names repeat across the city many times over.
The altitude
2,240 metres, which affects most visitors for a day or two: breathlessness on stairs, poorer sleep, and alcohol hitting noticeably harder. Plan a light first day.
The ground
Soft, saturated clay that compacts as groundwater is pumped from beneath it. The city is sinking — parts of the centre by more than ten metres since the conquest, some outer districts by up to 40 centimetres a year.
The same clay amplifies earthquakes arriving from the Pacific coast, which is why 1985 and 2017 were so destructive.
The climate
Subtropical highland — mild all year and cool at night because of the altitude.
Dry season October to April, sunny and pleasant, with cold mornings in December and January. Rainy season June to September, which means bright mornings and a heavy afternoon downpour.
The language
Spanish, and English is much less widely spoken than visitors expect outside hotels, the fashionable neighbourhoods and the main museums. A little Spanish goes a long way and is appreciated.
Nahuatl survives in a very large number of everyday words — chocolate, tomato, avocado, chilli, coyote — and in place names across the city: Chapultepec, Coyoacán, Xochimilco, Mixcoac.