An ideal bike frame is lightweight and sturdy. Frame sizes are measured in inches or units, or sizes S, M, L, XL. Bicycle frames are typically made from steel, aluminium or carbon fibre. Steel frames were once common but are now in decline; while being strong and relatively cheap, they are heavier and subject to rust. Alloy frames are cheaper but heavier than carbon fibre frames, which offer a lighter, more flexible, and yet equally strong option.