In less than a week since it was unveiled, a painting of Michelle Obama that hangs in the National Portrait Gallery has provoked much discussion. Commentators have pointed out notable features of artist Amy Sherald’s figure: the former first lady’s dress, her skin color and even her nails. But they missed the most remarkable detail of Obama’s portrait: her uncovered arms.

This is not the first portrait of Obama without sleeves.

Her official photographic portrait, released in 2009 and set in the White House’s oval-shaped Blue Room, where she is framed by a window facing the South Portico, pictures her posed with her left hand resting on a lily-and-tulip-laden marble table. Smiling, she appears relaxed and ready to extend her right hand in a welcoming gesture appropriate to a space associated with receptions and receiving lines.

That portrait also invoked the history of the White House. Situated just over the first lady’s left shoulder is a provocative prop, Rembrandt Peale’s portrait of the nation’s third president, Thomas Jefferson, captured just slightly out of focus.

The composition was a powerful reminder of how the meaning of a black woman in the White House had changed over the course of American history. The nation’s first black first lady stood in front of one of the scions of American slavery and American freedom, and his shadowy presence only underscored how black women have come to take charge of their political identities. ...