Why Are These Ring Structures Considered Diastereomers?

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NY152
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Could someone explain to me why the ring structures in this photo are diastereomers? From looking at them, I see a plane of symmetry, and no chiral centers so I'm unsure how they're diastereomers without have R/S characteristics
 

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They are cis-trans or geometrical isomers. I believe these days geometrical isomers are classified as a subset of diastereomers. Chiral atoms are not necessary for diastereoisomerism.