Garry Winogrand, above, made crooked horizons work.
However, crooked horizons, don't work, most of the time.
Crooked horizons are especially distracting when there's:
• Architecture in the scene.
• Other man-made things with straight lines and right angles, such as a formal garden.
• Obvious horizon lines, such as the shoreline of a lake.
On many cameras, you can display a tick-tack-toe grid.
This helps you to position the camera's horizon line along the horizon line in the scene.
You can also use the grid to apply the Rule of Thirds, as needed.
Some cameras have a built-in level that you can display on the LCD screen.
Some tripods have bubble levels, and you can place one in the hot shoe on top of your camera.
The hot shoe is where a separate flash is placed.
Levels may be helpful, such as when you're facing, say, the ocean.
But, when you point your camera down the beach, the horizon line of the level camera won't match the one in the scene.
When photographing a building, you often have to aim your camera up at a building to get it in the frame.
But, when the camera is no longer parallel to the building, it:
• Converges like railroad tracks receding in the distance.
• Falls backwards.
To avoid the above, photograph the building from another building.
By doing so, the camera is parallel with the subject building.
Or, use a perspective control (PC) lens.
A PC lens allows you to keep the camera parallel to the building.
The above two solutions work for architectural photographers, but don't work for most photographers.
You can rotate the photograph to correct the horizon line.
The photograph will have to be cropped, however.