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With the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter and a Thunderbolt cable, you can use the LG UltraFine 5K Display at 4K and lower resolutions with these Mac models: You can use the LG UltraFine 5K Display with these Mac models at full 5K resolution:

Compatible Mac models are grouped below by the resolutions that they support. The LG UltraFine 5K Display requires a compatible Mac running macOS Sierra 10.12.1 or later (macOS Sierra 10.12.2 or later is recommended). System requirements and compatible Mac models The display has three downstream USB-C ports (5 Gbps) that offer additional connectivity and power to compatible devices and accessories.

When you connect the display using a single Thunderbolt 3 cable (included), it provides up to 85W of charging power to your MacBook Pro with Thunderbolt 3 (USB-C) ports.

Integration with macOS lets you control your display's volume and brightness without the need for physical buttons on the display. The 27-inch LG UltraFine 5K Display features a 5120 x 2880 resolution, a P3 color gamut, 500 cd/m2 of brightness and built-in stereo speakers, camera, and microphone.

High sierra web cam works once and then not again