The hydrogen gas in the Milky Way is organized in a rather thin disk. With a diameter of perhaps 30 or 40 kpc, the disk has a thickness of about 300 pc. In this disk, hydrogen is concentrated roughly in spiral arms. However, as seen at galactic longitude 90º the more distant spiral arms do not lie in the Galactic Plane, but slightly above it, as is seen in the deprojected view:

What happens on the opposite side, at logitude 270º? This cannot be observed from central Europe, but very nicely from the southern hemisphere:

These data from GCO-SRT, also deprojected, show that the outer spiral arms now lie below the Galactic Plane.

Thus, the outer parts of the Milky Way are slightly tilted with respect to the inner parts, like the rim of a slouch hat!

Already in the radial velocity-latitude maps this is can be seen: