Our Callan brand deadbolts come in the single cylinder and double cylinder options. The single cylinder uses a key to lock/unlock the door from the outside and a thumb lever to lock/unlock the door on the inside (most popular). The double cylinder function requires a key to lock/unlock the door on either side of the door (rare, but helpful in some applications). Our deadbolts come in a variety of finishes and custom key-alike options may be selected during the ordering process.