in diameter, and commonly resemble bicycle wheels. Wheels are rubber-tired and may be solid, pneumatic or gel-filled. The wheels of folding chairs may be permanently attached, but those for rigid chairs are commonly fitted with quick-release axles activated by depressing a button at the centre of the wheel. All major varieties of wheelchairs can be highly customized for the user's needs. Such customization may encompass the seat dimensions, height, seat angle, footplates, leg rests, front caster outriggers, adjustable backrests and controls. Various optional accessories are available, such as anti-tip bars or wheels, safety belts, adjustable backrests, tilt and/or recline features, extra support for limbs or head and neck, holders for crutches, walkers or oxygen tanks, drink holders, and mud and wheel-guards as clothing protectors. Light weight and high costs are related to the manual wheelchair market. At the low-cost end, heavy, folding steel chairs with sling seats and little adaptability dominate. Users may be temporarily disabled, or using such a chair as a loaner, or simply unable to afford better. These chairs are common as "loaners" at large facilities such as airports, amusement parks and shopping centers. A slightly higher price band sees the same folding design produced in aluminium. Price typically then jumps from low to mid hundreds of pounds/dollars/euros to a four figure price range, with individually custom manufactured lightweight chairs with more options. The high end of the market contains ultra-light models, extensive seating options and accessories, all-terrain features, and so forth. The most expensive manual chairs may rival the cost of a small car. Manual attendant-propelled wheelchairs An attendant-propelled wheelchair is generally similar to a self-propelled manual wheelchair, but with small diameter wheels at both front and rear. The chair is maneuvered and controlled by a person standing at the rear and pushing on handles incorporated into the frame. Braking is supplied directly by the attendant who will usually also be provided with a foot- or hand-operated parking brake. These chairs are common in institutional settings and as loaner-chairs in large public venues. They are usually constructed from steel as light weight is less of a concern when the user is not required to self-propel. Specially designed transfer chairs are now required features at airports in much of the developed world in order to allow access down narrow airliner aisles and facilitate the transfer of wheelchair-using passengers to and from their seats on the aircraft. Powered