A passenger booked through to a final destination can find the allowance changing between legs. Two different rule systems exist, and which applies depends on the itinerary.
Two conventions govern the question
One approach applies the rules of the carrier operating the most significant portion of the journey to the entire itinerary. The other applies each operating carrier's rules to its own leg.
Which applies depends on the regions the journey touches. Itineraries to, from or within certain areas fall under one convention, and the rest fall under the other.
Neither is universal, and airlines publish their interpretation in their conditions of carriage. The gap between the two is where most disputes originate.
Weight and piece systems collide
Some regions apply an allowance by total weight, and others by a number of pieces each within a weight limit. Journeys crossing between them must resolve the two.
A traveller compliant on the first leg can be over the limit on the second. Charges are then assessed at the connecting airport, often at higher rates than an advance purchase.
Cabin baggage rules diverge more sharply than checked ones. Dimensions, weight and the number of items allowed vary considerably between carriers on the same alliance.
Separate tickets remove the protection entirely
When legs are booked as separate tickets, there is no single itinerary to interpret. Each ticket is a distinct contract, and each carrier applies its own rules from scratch.
Baggage must usually be reclaimed and rechecked, which exposes it to a fresh weighing. Charges apply again on the second ticket.
A missed connection on separate tickets is also the passenger's problem rather than the airline's. The saving on the fare is real and so is the risk.
Status and card benefits follow their own rules
Additional allowances from loyalty status generally apply where the granting alliance operates the flight. A leg on a partner outside the alliance may not honour it.
Benefits attached to payment cards typically apply only to flights booked with that card on specified carriers. They are contractual rather than part of the ticket.
Because these sit outside the conventions, they are the first thing to disappear on an itinerary combining several operators.
Checking before departure
The allowance printed on the booking confirmation reflects the rule the issuing system applied. It is the document to present if a station applies something different.
Weighing bags at home removes the most common failure. Airport scales are the first point at which a discrepancy becomes expensive.
Where an itinerary mixes carriers, confirming the allowance for each leg individually takes minutes. It is considerably cheaper than resolving it at a connection.