High density foam plus crater holes = extreme protection minus the bulk. Crater holes help circulate air to cool your laptop when carrying it. Low-profile, lightweight design. Airport security friendly: leave it in the sleeve. XS - Fits 11" MacBook Air & smaller, S - Fits 13" MacBook Pro & smaller, M - Fits 15" MacBook Pro & smaller, L - Fits 17" MacBook Pro & smaller, XL - Fits laptops larger than 17" MacBook Pro Keeps bad stuff at bay, but lets the air flow through.