Hi. It's only called doubly periodic because two directions are sufficient to cover the entire plane. You need atleast two directions to cover the entire plane ( they can be either +30 degrees and +90 degrees from the horizontal line or you can also choose -30 and +90 from the horizontal axis or you can choose any other two directons which can tile up the entire plane). The exact vectors used are not important but what is important is that you need "two" of them to cover the entire plane.
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