The altitude
2,240 metres. Expect to be breathless on stairs for a day or two, to sleep poorly the first night, and for alcohol to hit noticeably harder.
Take the first day gently and drink more water than usual.
The taxi rule
Never hail a taxi in the street. Use Uber, DiDi or Cabify, or a sitio rank, or an authorised airport taxi bought at a booth inside the terminal.
This is the single most important practical rule in the city.
The water
Do not drink the tap water. Bottled and purified water is universal and every hotel provides it. Restaurants use purified water for ice and for washing produce as standard.
Ask for agua purificada.
Earthquakes
Download the SkyAlert or 911CDMX app. The city’s seismic alert can give up to 60 seconds’ warning before shaking, and it is broadcast from street speakers as a rising alarm.
If it sounds: leave the building if you can do so quickly, otherwise stand in a marked zona de seguridad away from windows. Never use a lift.
Full detail in altitude, safety and the shaking ground.
Safety, honestly
Violent crime against visitors in the central districts is uncommon, and the cartel violence that dominates international coverage is concentrated elsewhere in the country.
What does happen is opportunistic: pickpocketing on crowded metro lines and at markets, phone snatching, and robbery in unlicensed taxis.
Carry little, keep nothing in a back pocket, use ATMs inside banks in daylight, and follow the taxi rule.
Money
The peso. Cards work in the central districts; carry cash for markets, street food and small restaurants.
Withdraw from bank ATMs inside branches. Decline any offer to charge you in your home currency.
Tipping
10 to 15 per cent in restaurants. Small change for supermarket baggers, car watchers, petrol attendants and musicians. Nothing at street stalls.
Air quality
The valley traps pollution, worst in the dry season from February to May. A contingencia ambiental restricts traffic on bad days. Asthmatics should carry an inhaler.
Immigration
Most nationalities enter visa-free. Mexico now stamps the permitted number of days in your passport rather than issuing a paper FMM, and the officer decides the number — it may be fewer than you expect.
Check the stamp before leaving the desk.
Odds and ends
A quesadilla in Mexico City does not automatically contain cheese. Ask con queso. This is genuinely true and a long-running national argument.
Museums close on Mondays, and so does Chapultepec park.
Plugs are North American two- and three-pin, 127V.
Emergency numbers
911 for everything. Locatel on 55 5658 1111 for lost property, missing persons and general assistance.