How do you name a plane?(math)?

Question

do you need a line on top of the letters or a square before?

Answer 1

I don't know that there is some formal convention, but often capital Pi is used: Π

Often, it has subscript:

Π₁

Π₂

Pi has other meanings though (ratio of diameter to circumference, product of a series). The subscript does help it seem less ambiguous though.

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You could also use P with subscript if you wanted.