This is an excerpt from Alpine Skiing.
Your boots are the most important decision you will make. You can have the best ski in the world, but if the connection to it is less than optimal, the transfer of movement and force will be wasted effort.
Fit and performance are the two most important factors in boot choice. Fit is the first item to consider. It is usually associated with comfort, but performance and even warmth are also affected by it.
Each foot contains 28 bones, 19 muscles, 33 joints, 31 tendons, and 107 ligaments. Accommodating all of these in a workable plastic shell is as much an art as it is a science. Boot fitters are well trained in theory and understand how the foot must work in conjunction with the ski boot and ultimately with the ski.
For a ski boot to transmit the action of the skier to the ski, it must be tight, meaning the boot is snug everywhere around the foot. Like a firm handshake, the boot must encase the foot with equal pressure around the entire foot. All skiers can point to places where they like their boots tight and where they say snugness does not matter. In general, the tighter the boot, the better.
Pressure points and hot spots are the nemeses of skiers. These are spots where the boot does not conform with the foot and creates an uncomfortable situation. The reaction of the skier, and the unskilled boot fitter, is to get a larger boot. This boot may be comfortable, but it will be a low performer, even if the boot is in a high-performance category.
Ski socks should be considered part of the ski boot. Socks are the interface between the skier's foot and the boot liner, and they actually become part of the liner in the fitting process. Always use socks designed for skiing. They are made of materials that wick, pad, don't rub, and are well thought out for skiing. The sock thickness is irrelevant because it depends on the rest of the fit. New boots should be fitted with thin socks. As the liner packs out, a thicker sock can be worn. Here is a list with definitions of the main features of a ski boot:
bail—The hook at the end of the buckle that fits into the ladder.
boot board or zeppa—The platform inside the boot shell on which the liner rests.
buckle—The lever that closes and tightens the shell.
cant adjustment—An upper-cuff lateral adjustment used to align the leg with the bottom of the boot. Not present on all ski boots.
cuff—The upper plastic part of the boot shell.
flex adjustment—A device used to adjust the forward tension, making the boot stiffer or softer. Not present on all ski boots.
footbed—The part of the inner boot that the foot stands on.
forward-lean adjustment—Changes the angle that the lower leg is pitched forward relative to the foot. Not present on all ski boots.
inner boot or liner—The soft inside of the boot.
ladder—The graduated rung attached to the opposite side of the shell that the bail fits into.
power strap—A band that wraps around the top part of the cuff.
ramp angle—The incline of the boot board.
shell—The plastic outer portion of the ski boot.
ski-walk adjustment—An adjustment that permits the ski boot to have its forward lean for skiing and then allows for a more upright shaft to make walking easier. Not available on all ski boots.
Read more from Alpine Skiing by Ronald Kipp.