That is explained in the article.
Benjamin YoungSavage

Thanks for your reply Benjamin. What source is there for the claim that during the Civil War, unionist Marylanders only used the yellow-and-black Calvert arms other than wiki >

The 1904 Act that made the MD flag official claims that it’s the flag used since colonial days by common consensus. Page 97 in that 1907 Maryland State document gives a similar history of the flag.

See as well a pre-Civil War painting from 1853 depicting colonial Marylanders that shows a hung quartered flag just as used today (here’s the painting and it’s dated here and here). That painting alone seems to bring the claims on the sos.maryland page into doubt. Notice that sos page cites no sources.

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