The left outside rear-view mirror should be adjusted, with a minimum head movement, so that you can see:

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The left outside mirror should be adjusted so you can just see the left rear edge of your own vehicle with only a small amount of head movement. This setup keeps your awareness of vehicles in the left lane while you’re looking straight ahead or scanning forward, reducing the need to twist your head to monitor traffic behind you. Seeing the left rear edge confirms where your car ends in relation to nearby vehicles, which helps you judge gaps and change lanes more safely. The horizon behind you isn’t the target of this mirror adjustment, and the mirror won’t reliably show the entire left lane or the right side of your vehicle from this position.