Yes, the Xbox Series X can be used horizontally as well as vertically

[Windows Central]

9000 hours in MS Paint reveals what a horizontal Xbox Series X might look like
Microsoft chose to debut the Xbox Series X in an upright almost PC tower-like fashion, but the monolithic console can be used horizontally as well.
Microsoft unveiled the next-gen Xbox dubbed the "Xbox Series X" during The Game Awards, and I've already seen a few interesting comments about the design.

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