There are two major type of backpacks: External frame, and internal-frame. With an external-frame backpack, aluminum bars form a frame outside of the bag. Externals have traditionally been used for serious hiking, with various add-ons, such as sleeping bags being lashed onto the frame. In addition, the bulk of the bag sits rather high on the back. Externals are not shaped or sized to be considered a carry-on and for this reason, are not recommended.
The internal-frame backpack has no visible metal frames, as these are all constructed within the bag. Those built for serious hiking are fully padded, with very wide waist belts to take most of the weight of the backpack off of the shoulder straps, making it more comfortable for long walks. Not all internal-frame backpacks are sized to fit as a carry-on. In addition, the wide, padded waistbelts often protrude from the body and gets in the way when you try to stash your backpack in the overhead or underseat compartment. If you choose an internal-frame backpack, take a look to see if the waistbelt can be tucked away or at least folded tightly near or around the body of the pack.
Another type of internal-frame backpack is the traditional rucksack or bookpack that you see students carrying. These are not padded as well as the rugged hiking type, but are considerably cheaper.
Bookpacks are a great way to go, especially if you can find a large one to carry everything you need. Some of them have side pockets that fit pairs of shoes or umbrellas. Most bookpacks comfortably fit under the seat, and they do not have too many straps to get in the way. They have the added benefit of being very comfortable to carry, because their shoulder straps are almost always padded. Some models also have padded backs for added comfort. Some even have waist straps, to distribute the load on your back well.
Important: If you decide to use a backpack, here is my sternest warning: Do not use a top-loading bag (the type with a cinch closure on the top only). You've seen them--they saw a real boon the last two or three years. The smaller ones were particularly popular with young women, who used them as purses. These packs are very difficult to find things in, because the opening is small and only at the top. You will end up messing up your nicely packed stuff because you can't find this or that. Use a bag with a fairly full C-shaped zipper all the way across the front.
Advantages: Easy to carry, easy to clean. Comfortable to walk with for long distances, easier to look casual in. Can serve double purpose by being a day pack once you reach your travel destination.
Disadvantages: You look like a low-budget traveler or a college student, and you can't really carry the backpack any other way besides one shoulder or two. Some models with lots of straps may catch on other things.
Important features to look for:
Bookpack/rucksacks come in two different zippered formats: Top-loaders and front-loaders. Front-loaders have a C-shaped zipper that allows you to unzip most, if not all of the flap open when the pack is lying with the straps down. These are great for accessing things in your pack, and you will have far more control over how you back.
The second style is the top-loader, which has a cinch strap on the top, which may or may not be covered by a small flap (which may have a snap or buckle that keeps it secured). While it may be harder to get to your things with a top-loader, this disadvantage also serves as its biggest benefit: This format makes it difficult to pilfer, and you do not have to worry about a busted zipper. And because you don't have to worry about a busted zipper, you can pack and squish down to your heart's content! Just make sure you leave remember where you squinched things into, because you don't want to stand on a street corner trying to pull out your umbrella in a sudden downpour! Whether you decide to use a front- or top-loader is a personal choice.
Next page: Travelpack Basics.