If you see the same thing on both displays, you should start by making sure the displays are Extended instead of Mirroring ...

Sure, your monitor looks nice out of the box, but you can make it even better.

A reader's El Capitan installation started blacking out one monitor when he's using apps full screen.

This guide will show you how to get fullscreen 21:9 content to display correctly on your ultrawide monitor. The issue with ultrawide video ...

To get your widescreen monitor to display with the proper aspect ratio, the most vivid colors and the sharpest possible picture, it's important to set it to its optimal display resolution.

In OS X Mountain Lion, Apple has taken an incremental step toward making OS X's new iPad-like Full Screen mode more useful: you can now pick which connected display you want to use.

By default, Windows 11/10 selects the best display settings considering screen resolution, monitor refresh rate, and color according to your monitor.