This is a sale for the right (R) side only of the WF-1000XM4. This is a used item, but the battery is a genuine Z55H battery and has been replaced with a new one. The item has been tested and cleaned. You don't have to worry about the battery degradation that is common with this model. Please check the photos for any scratches or stains. We have confirmed the operation, but if there are any initial defects, we will accept returns if you contact us before leaving feedback. For those who have a broken or lost one side, please consider this. The version is the latest 2.3.1. Pairing with version 2.1.0 has been confirmed, but pairing with earlier versions is unknown. Please check your current version. Pairing instructions for those who purchased only one side: Place both left and right earbuds in the case and insert the USB cable. It will blink red rapidly three times, then press and hold the touch sensors on both the left and right earbuds (for about 10 seconds) until it blinks red slowly. Once it blinks red slowly, release your fingers. After it blinks green four times, initialization and pairing are complete. Delete the previous device information from the Bluetooth settings of the connected device and re-register.