British Airways is shrinking the size of carry-on baggage that passengers are allowed to take on flights.

Beginning August 18, the airline will decrease the size allowance for passengers' second carry-on bag from 18 x 14 x 8 inches (45 x 36 x 20cm) to 16 x 12 x 6 inches (40 x 30 x 15cm). The size allowance for passengers' main carry-on will stay the same: 22 x 18 x 10 inches (56 x 45 x 25cm).

Changing the second carry-on allowance may seem like a small thing, but the battle over what passengers can and cannot bring on to the plane with them can get heated quickly. Travellers are fiercely protective of their carry-ons.

In June, when the International Air Transport Association guidelines that would significantly decrease the standard allowance (as in, you'd have to buy new luggage), the reaction was swift.

First, from travellers wary of changes that would make it even more difficult to avoid checking baggage, and then from airline executives themselves, who (to their credit) don't want to further inconvenience passengers. 

However, British Airways is taking steps to rein in excess baggage brought on board by passengers, and the change is likely intended to ensure that any second bags will fit underneath seats instead of requiring overhead bin space. When everyone on a flight brings several bags with them, precious bin space fills up quickly — and that can delay departures and even cause fights between passengers. 

British Airways typically does not charge a fee for the first checked bag for economy passengers, although there are "hand baggage only" fares, meaning passengers save some money if they don't check anything. 

Meanwhile, U.S. carriers have seen an understandable increase in carry-on baggage since the widespread introduction of fees for even a single checked bag.

To find out exactly how large a bag you can bring on to your next flight, check out our guide to what different airlines around the world allow:

(Content and images source: mashable.com)