as in the picture, I received 2 pieces plus the "central unit". I then added a third thermostat. The "central" i.e. the square box communicates with the thermostats via 868 MHz radio, and with the WLAN on 2.4 GHz. Attention: The system does not like short (seconds) power interruptions. However, the central unit can also be supplied directly with 5 V via USB. via USB. A micro-USB plug is well hidden under the 230 V connector.