Airline passengers may be surprised to learn what items can legally be brought onboard a plane as long as the luggage is checked, not carried on. Essentially, anything that is not flammable, explosive or a direct threat to the crew or passengers can be stored somewhere on an airplane. This doesn't mean a passenger will have access to that item during flight, however.
Airlines are generally in the customer transportation business, not law enforcement or customs. Passengers are advised on what items are strictly forbidden in carry-on luggage and what must be stored in the checked baggage hold, but they are still free to pack clothing, electronics, personal care items, souvenirs, gift items, paper goods and so on.
As long as the item is legal to possess and not considered a threat to public safety, it will most likely be allowed on an airplane. The real trick for passengers is clearing the item through customs once they have deplaned.
Recent changes in airplane security procedures have created some confusion as to what types and amounts of liquid items can be brought onboard a plane in carry-on luggage. Generally speaking, there is a three ounce rule in effect, along with a requirement that liquid items be stored in transparent quart-sized plastic bags.
This rule also included beverages brought in from home or an outside source. Drinks purchased in the passenger-only areas of an airport, however, are not restricted.
Personal entertainment items such as electronic games, music players and laptop computers can also be carried onto a plane, but passengers who used these devices may be asked to use earphones or set the device to a silent mode. Cellphones can be carried onto an airplane, but their actual use during flight is often discouraged.
Personal comfort items such as travel pillows and eye masks can also be brought onboard an airplane, but those items must be storable under a passenger's seat or inside an overhead bin.
Individual airlines often publish extensive lists of acceptable and prohibited items on their official websites, and this general information can also be found on travel-oriented websites and guidebooks for tourists.