A quirk of manga/anime art where a character's face is viewed from the side, yet their mouth is drawn on the side of their face, rather than on the front.
Often the lips are still drawn on the front of the face, making it appear as if the character has two mouths.
One reason this may be done is to show more of the character's facial expression when it wouldn't otherwise be visible. Another reason is that, in animation, it's faster and easier to draw the mouth on the side than to animate the entire mouth opening normally, which would include having to adjust the jawline and lips.
For mouths that are literally sideways (rotated from the position of a regular mouth by 90°), use vertical mouth.
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