The First Alert Precision Detection 2-in-1 Combination Smoke and Carbon Monoxide Alarm Set is a reliable and essential home security device. With dual functionality, this alarm set detects both smoke and carbon monoxide, providing peace of mind for you and your family. As a trusted brand in home improvement, First Alert ensures quality and dependability in their products, making this alarm set a valuable addition to any home. This includes 4 alarms. One is new open box and rest are unopened. 3 of them are model SMCO210 w 10 year battery and one is SMCO200 with replaceable batteries