The 180 degree rule is something that is taught mostly in film school but applies to other visual story telling mediums, like comics. Imagine a line of string that connects two characters in a scene; this string is the Axis Line. If the camera stays on one side of the Axis Line, the first character is always Frame Right and the second character is always Frame Left no matter which angle the camera shoots from. This is important because it means that even if you completely change the appearance of a character between shots, the audience still knows who's talking because the Axis Line was established and the first character is always Frame Right and the second character is always Frame Left.
A perspective grid is like a 3D map of a scene. It's easier to show than to explain:
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