If you use the Plug-In Wireless Chime or the Battery-Operated Wireless Chime to receive audible notification when a visitor arrives at your doorstep, you can expand this wireless doorbell system by mounting an Additional Wireless Chime Transmitter at another entry to your home. Just mount this battery-powered doorbell at your front door, and when a visitor presses it, it transmits wirelessly up to 100 feet to the chime receiver in your home. The Wireless Chime kits are sold separately. Additional Wireless Chime Options Available If you're just getting started with this wireless doorbell system and know that you'll need two doorbells, the 2-Entry Plug-In Wireless Chime and the 2-Entry Battery-Operated Wireless Chime kits each come with two transmitters and a single plug-in chime that rings different tones to indicate where your guest is. This Additional Wireless Chime Transmitter will not trigger a different tone.