You will never leave your bike lock keys at home again, because you will not need keys anymore... this is a combination lock! The dimensions of the "U" area are 8 inches by 4 inches. 12mm hardened HIGH-GRADE CARBON ALLOW STEEL SHACKLE resists hand tools. 4-digit RESETTABLE COMBINATION.This lock includes eligibility for $1,750 of Kryptonite's Anti-Theft Protection Offer (the anti-theft protection offer is optional and you must register within 30 days of purchase to participate in the program - details are in the package and on the Kryptonite website). Included with this lock is eligibility for the Kryptonite Combo Safe Program... you can register your pre-set or resettable combination with Kryptonite for free. If you do happen to forget your combo and registered it with uKryptonite, contact them and they will be happy to provide it to you.For over 40 years, Kryptonite has been the leader in action sports security. As the creator of the original bicycle u-lock in 1971, Kryptonite has set the standard for premium quality security products.