“principal plane”

We have one entry that includes the term principal plane.

principal plane of symmetry

: a plane of symmetry in a crystal that includes two or more axes of symmetry

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Play Quordle: Guess all four words in a limited number of tries. Each of your guesses must be a real 5-letter word.
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Play Blossom: Solve today's spelling word game by finding as many words as you can using just 7 letters. Longer words score more points.
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Another Great British Vocabulary Quiz, Innit? A double-decker quiz with words from across the pond