If you're heading to Mexico on holiday, one small document — or increasingly, just a passport stamp — will shape your entire stay. It's called the Forma Migratoria Multiple, or FMM, and it's the visitor permit issued to passport holders from visa-exempt countries staying up to 180 days for leisure, business or volunteering.

The big change: paper FMMs are being phased out. Where you once filled in a two-part form and kept the stub for departure, immigration officials at many airports now simply stamp your passport, with your allowed number of days recorded electronically against your passport number. If you fly in, keep an eye on your ticket's fine print — the roughly US$35 permit fee is usually bundled into your airfare's surcharges. Land arrivals get a small perk too: enter by road and leave within seven days and the permit is free. You can also apply online in advance via the INM's eFMM system and bring a printed copy to be stamped at the border.

The rules around your stay are stricter than many travellers assume. The immigration official at entry writes a number of days on your permit — never more than 180, but often less — and that's final. There's no extension or renewal. Count your exit date from the stamped arrival date, not from when your holiday plans end.

Leave Mexico and re-enter on a new FMM and your 180 days technically reset, with no required minimum time out of the country and no cap on yearly re-entries. But don't treat this as a loophole for an indefinite stay. With computerised entry and exit records, officials can see your movement history, and so-called 'perpetual visitors' hopping out and back in every few months are increasingly being questioned — and in some cases turned away — at the border. If Mexico is your long-term plan, the sensible route is applying for residency through a Mexican consulate abroad.

Practical details worth knowing: airlines won't let you board an international departure unless you surrender a paper FMM or show your entry stamp. Leaving by land, hand the form in voluntarily — hanging onto it can cause delays on your next visit. Lose a paper permit and you'll need to visit an immigration office, fill in paperwork and pay around US$60 for a replacement before flying out. Overstay your granted days and a per-day fine awaits at the airport kiosk or immigration office before you can leave. And whatever you do, don't alter the number of days written on the form — the serial number is tied to an electronic record of your entry, and tampering won't end well.

For most holidaymakers, this is all refreshingly straightforward: arrive, get stamped, keep track of your dates, leave on time. The paperless shift actually removes the classic 'I lost my FMM stub' panic. But if you're dreaming of a full winter in Oaxaca or a slow year on the Pacific coast, take the re-entry caution seriously — repeated FMM runs are a gamble with your future entry. For anything beyond a couple of stints, residency is the cleaner answer.

Details on the FMM and its phase-out come via Mexperience.