This section explains the hours of service (HOS) rules, and outlines Subpart A—General and Subpart B—Electronic Logging Devices (ELDs). “Hours of service” refers to the maximum amount of time drivers are permitted to be on duty including driving time, and specifies number and length of rest periods, to help ensure that drivers stay awake and alert. In general, all carriers and drivers operating commercial motor vehicles (CMVs) must comply with HOS regulations in 49 CFR 395. Exceptions are listed in Sections 395.1 (b through x). FMCSA's Interstate Truck Driver’s Guide to Hours of Service provides detailed examples of how the HOS rules are applied, and how logbooks should be completed.