Everyone knows the cabin rules. Liquids in a bag, laptop out of the case, no knives. The hold feels like the forgiving place, the hole in the aeroplane where the awkward things go once the security theatre is over.

It is the opposite. For an entire class of ordinary objects the hold is the stricter of the two, and the reason is physical rather than bureaucratic. A fire in the cabin has somewhere between 100 and 400 people sitting next to it, several of whom are trained to put it out and are holding an extinguisher within ten seconds. A fire in the hold has nobody. The door is locked, the compartment is pressurised and sealed, and the only thing that will ever touch that fire is whatever the aeroplane can spray into it by remote control from the flight deck.

That single asymmetry explains almost every rule below, including the one most travellers get backwards: the spare battery in your pocket is legal, and the identical spare battery in your suitcase is not.

The rule underneath all the other rules

An airliner's belly is not one undifferentiated space. Since 1964 the American certification standard, 14 CFR 25.857, has sorted cargo and baggage compartments into classes according to a single question: what can anybody actually do about a fire in there?

A Class B compartment is one where, in the regulation's own words, there is “sufficient access in flight to enable a crewmember, standing at any one access point and without stepping into the compartment, to extinguish a fire occurring in any part of the compartment using a hand fire extinguisher”. Somebody can reach it. A Class C compartment is defined by what it has instead of that reach: a smoke or fire detector wired to the flight deck, “an approved built-in fire extinguishing or suppression system controllable from the cockpit”, a way of keeping smoke and extinguishing agent out of the cabin, and a way of controlling ventilation so the agent is not simply blown away.

Read that list again and notice what is missing. There is no person in it. A Class C hold is fought by a bottle of gas and a switch, and by nothing else, for as long as the flight lasts.

There used to be a sixth category, Class D, and it was the one that made the hold genuinely dangerous. A Class D compartment had no detector and no suppression system at all. The design assumption was that the compartment was sealed tightly enough that a fire would consume the available oxygen and go out on its own. On a short sector, with ordinary luggage inside, that assumption mostly held.

It stopped holding on 11 May 1996, when a DC-9 went into the Everglades with 110 people aboard after a consignment of chemical oxygen generators ignited in the forward hold. Oxygen generators make their own oxidiser. A sealed box was precisely the wrong place to put them.

That last sentence of the FAA's is the uncomfortable one, and it is the reason the rest of this article matters. The shipment was already illegal. Prohibiting a thing does not keep it off the aeroplane; it only determines who is at fault afterwards. The regulator's response was therefore not another prohibition but a change to the aircraft itself.

Amendment 25-93 eliminated Class D as an option for new type certification and required existing Class D compartments in passenger service to be brought up to Class C detection and suppression standards by 19 March 2001. The FAA's own analysis put the affected fleet at 2,991 passenger aeroplanes and 313 all-cargo aeroplanes across 21 transport-category types. The airlines objected to the three-year timetable on cost grounds and the Air Transport Association told the docket that it did not think “the marginal added benefits resulting from a 3-year schedule justifies the extra cost”. The FAA kept the three years.

The practical result is that the hold under your feet today almost certainly has a smoke detector and a halon bottle. What it still does not have is a person.

What the hold forbids that the cabin allows

This is the category that catches people out, and it is almost entirely about batteries.

Under 49 CFR 175.10(a)(18), spare lithium cells and batteries “must be carried in carry-on baggage only”. The international rule is identical in effect: IATA's Table 2.3.A lists spare and loose batteries, and states explicitly that “articles which have the primary purpose as a power source, e.g. power banks are considered as spare batteries”. A power bank is not a gadget. It is a battery with a socket on it, and it travels in the cabin or it does not travel.

The limits themselves are generous. A lithium-ion battery of up to 100 Wh needs no permission at all; between 100 Wh and 160 Wh you need the operator's approval and may carry two. Lithium-metal batteries are capped at 2 grams of lithium content, rising to 8 grams for portable medical devices with the operator's agreement. IATA adds numbers the American text does not: a maximum of 20 spare batteries and 15 portable electronic devices per person, both of which an operator may waive upward.

The same cabin-only logic applies to things most travellers would never classify as batteries. Electronic cigarettes, vapes and every other “battery-powered portable electronic smoking device” must be on your person or in carry-on, and the regulation adds that “recharging of the devices and/or the batteries on board the aircraft is not permitted”. One packet of safety matches or one lighter may be carried on your person; lighter fuel and refills are banned from the cabin and the hold alike. Smart suitcases with built-in batteries must be carried into the cabin unless the battery comes out, and may only be checked with the electronics live if the cell is trivially small — under 0.3 grams of lithium metal or 2.7 Wh.

Devices you are allowed to check, such as a laptop, carry a condition that is widely ignored: they must be “completely switched off (i.e., not in sleep or hibernation mode)” and protected against being switched back on by the luggage shifting around them.

The FAA's own explainer on why lithium batteries behave the way they do, from the agency that writes the rule.

Why the ban exists in that direction and not the other

It would be reasonable to assume that spare batteries are banned from the hold because the hold has no suppression system. It has one. The real answer is that the suppression system does not work on this particular fire, and the FAA has said so in writing.

The same alert explains what happens next. A cell in thermal runaway generates enough heat to drive the cell beside it into runaway as well, so a fire that starts with one battery does not stay one battery. In a lithium-metal cell the result is a release of flammable electrolyte mixed with molten lithium and a pressure pulse, which the FAA warns can “easily damage (and potentially perforate) cargo liners, or activate the pressure relief panels” — and a breached compartment leaks its halon away, at which point the aeroplane has neither containment nor agent.

In the cabin, none of that chain gets started unobserved. A smoking device in a seat pocket is noticed in seconds, there is a crew member with a halon extinguisher and a thermal containment bag within a few paces, and the problem ends as a diversion rather than a hull loss. Put the identical battery in the hold and the first indication anybody gets is a smoke light on the overhead panel, by which point the only available response is to discharge the bottle and land as soon as possible.

That is the whole of the reasoning. The rule is not about the battery. It is about who is standing next to it.

What the hold allows that the cabin does not

The asymmetry runs both ways, and the items in this direction are stranger.

Small arms ammunition for personal use is permitted in checked baggage only, “if securely packed in boxes or other packagings specifically designed to carry small amounts of ammunition”, with clips and magazines boxed as well. The international rule is more explicit about quantity: Division 1.4S only, UN 0012 or UN 0014, and not more than 5 kg (11 lb) gross per person, with the added instruction that two passengers may not pool their allowances into one package.

A mercury medical or clinical thermometer — an object that would be confiscated at the security checkpoint without discussion — may travel, one per person, in its protective case, in checked baggage. Mercury attacks aluminium alloy, which is why it is nowhere near a passenger cabin, and why the rule insists on the case.

Alcohol is the one most people have met. Below 24% ABV there is no restriction at all. Between 24% and 70% you may carry 5 litres (1.3 US gallons) per person in unopened retail packaging, in either the cabin or the hold. Above 70% ABV it is not a beverage as far as the regulation is concerned, and it does not fly.

Then there is the item where the two rulebooks genuinely contradict each other. Under 49 CFR 175.10(a)(9) an American passenger may put one self-defence spray of no more than 118 mL (4 fl oz) in checked baggage, provided it “incorporates a positive means to prevent accidental discharge”. Under IATA's Table 2.3.A, “disabling devices such as mace, pepper spray, etc. containing an irritant or incapacitating substance are forbidden on the person, in checked and carry-on baggage”. Not restricted. Forbidden, in capitals, in the only column the table provides.

If you are flying a US domestic sector under US rules, the spray is legal in the hold. If the same suitcase is checked through onto an international flight, it is not. The table closes with the sentence that resolves every argument of this kind: the provisions “may be limited by State or operator variations”, and passengers should check with their airline. The hold is governed by whichever rulebook the operator is flying under, and that is not always the one printed in the country you are standing in.

The BBC follows a suitcase from the check-in desk to the aircraft, which is the part of the journey the rules above are actually written about.

The aerosol nobody worries about

Buried in the 1998 rulemaking is a hazard that has nothing to do with batteries and still has not gone away. When the FAA was arguing with regional airlines about whether small compartments should be exempt, it made a point about ordinary consumer aerosols that is worth reading slowly: in a cargo compartment, “the capability of the liners to withstand the penetration of flames is a secondary concern because the integrity of a liner can be destroyed by the force of an exploding aerosol can regardless of its capability to resist flame penetration”.

The liner is the fire-resistant skin that keeps a hold fire inside the hold. A can of deodorant that cooks off in a fire can tear it open, and the compartment stops being a compartment. This is why the aerosol allowance is a quantity limit rather than a blanket permission: medicinal and toiletry articles including aerosols may go in either the cabin or the hold, but no more than 2 kg or 2 litres in total per passenger, and no single container above 0.5 kg or 500 mL, with the release valve capped against accidental discharge.

The same reasoning sits behind the restrictions on things that contain their own gas cartridge. Self-inflating life jackets are permitted, with the operator's approval, at no more than two devices with two cartridges each plus two spares. A hair-styling iron with a butane cartridge may go in the cabin, one per person, with the safety cover over the heating element — but the spare cartridges for it are banned from both the cabin and the hold, because a loose cartridge is just a small pressure vessel with nothing around it.

How the detection and suppression architecture actually works on a modern airliner, walked through by a line pilot.

Two rulebooks, not one

It is worth being clear about what the lists above are and are not. They are safety regulations, written by aviation regulators, about what a substance will do in an aircraft. They are not security rules. The checkpoint rules that take your water bottle away are a separate body of law with a separate purpose, and the two overlap only by coincidence. A mercury thermometer is a security non-issue and a dangerous good; a 150 mL bottle of shampoo is the reverse.

Both of the safety texts are also written the same way round, and it is the opposite of how passengers read them. IATA's table opens by stating that “dangerous goods must not be carried in or as passengers or crew, checked or carry-on baggage, except as otherwise provided below”. The American regulation is a section of exceptions. Neither is a list of banned items with everything else permitted. They are lists of permitted items with everything else banned.

Which is the honest answer to the question people actually ask. If what you want to pack is not on the exception list — the airbag inflator out of a steering wheel, the camping stove with fuel still in it, the attaché case with a pyrotechnic anti-theft charge in the lid — the absence is the answer. The hold is not a loophole. It is the one part of the aeroplane where nobody can help you, and the rules are written as though that mattered, because it does.

Sources: 49 CFR 175.10, Exceptions for passengers, crewmembers, and air operators (eCFR); 14 CFR 25.857, Cargo compartment classification (eCFR); FAA final rule, Amendments 25-93 and 121-269, Revised Standards for Cargo or Baggage Compartments in Transport Category Airplanes, 63 FR 8032, 17 February 1998; FAA Safety Alert for Operators 10017, Risks in Transporting Lithium Batteries in Cargo by Aircraft, 8 October 2010; IATA Dangerous Goods Regulations Table 2.3.A, 67th edition, 1 January 2026; FAA Lessons Learned from Transport Airplane Accidents, ValuJet Flight 592, N904VJ.