Direct wire-in with 9V battery backup. Hardwired with battery backup for protection during power outages. Perfect mount for easy installation. Mounting bracket keeps the alarm secure over a wide rotation range for ideal alignment. Latching alarm indicator, visually identifies unit that initiated alarm or low battery. Interconnects with most First Alert and BRK hardwired alarms. When one alarm sounds, all compatible alarms sound (up to 12 smoke alarms and 6 other alarms). Test/Silence Button, temporarily silence or test the alarm with one-touch easy access button. Dust cover, keeps the alarm clean during construction.