The AST1 (add model) & the AST2 (add/subtract) are the new and hotest electronic tally counters on the market. They have an upgraded 2 year battery life and a backlight for dark situations. The E2 (add model) and the E3 (add/subtract) electronic tally counters have a battery life of 250 days. The AST2 and the E3 model are perfect for counting people going in and coming out. Both have large buttons for easy counting. All counters are silent when counting. Beepers can be turned on or off. Extremely accurate! Light weight and comfortable size to hold in you hand. Each model comes with a handy lanyard. The E2 and the E3 are slightly smaller than the AT1 and AST2.