Secretary of the Treasury Jack Lew announced on Wednesday that the Bureau of Engraving and Printing would replace the portrait of Alexander Hamilton on the $10 bill in favor of one featuring both Hamilton and a woman to be named later.

"I'm proud to announce today that the new $10 bill will be the first bill in more than a century to feature the portrait of a woman," Lew said in a statement on YouTube. "This historic endeavour has been years in the making."

Lew will decide by the end of the year which woman will share the bill with Hamilton. The new version of the bill will appear in 2020, the 100th anniversary of the 19th Amendment, which awarded women the right to vote.

The only legal criterion for who should be on the bill is that the person be dead. But the Treasury told The New York Times that Lew was looking for a woman "who was a champion for our inclusive democracy."

The push to put a woman on the printed US currency has been in progress for some time. In March, the organization Women on 20s began asking the public to vote for top female candidates to replace President Andrew Jackson on the $20 bill.

Among the 15 women included in the vote were Rosa Parks, Eleanor Roosevelt, Clara Barton, and Harriet Tubman. In May it was revealed that Tubman edged out Roosevelt with almost 34% of the vote.

It seems strange, though, that the Treasury chose not only to not replace Jackson on the $20 bill, but to not replace Hamilton either. Instead it chose to sidestep the matter entirely, having Hamilton share his portrait with a reputed female figure. It's unlikely to satisfy those in groups such as Women on 20s.

If Lew agrees that Tubman is the best candidate for the bill, however, she would become he first black person to be the face of an American paper currency and the first woman in more than a century.

Martha Washington was featured on $1 silver certificate in 1891 and Pocahantas was $20 national bank notes in 1863. Lewis and Clark's expedition guide Sacajawea and women's suffrage advocate Susan B. Anthony have been featured on unpopular US dollar coins.

"This is a way to literally pay respect to women that is long overdue and can be seen as a step in the right direction toward greater gains in gender and racial equality," Women on 20s executive director Susan Ades Stone told Business Insider in May.

Sen. Jeanne Shaheen (D-New Hampshire) introduced legislation in April to put a woman on the $20 bill. In a statement on the Treasury's announcement she sounded thrilled.

"While it might not be the $20 bill," she said, "make no mistake: This is a historic announcement."

The choice to upend Hamilton on the $10 bill as opposed to replacing Jackson on the $20 bill is a curious one.

There were clear reasons to replace Jackson on the bill. Jackson has long been reputed as a deeply flawed character, who owned hundreds of slaves, executed American soldiers for desertion, and oversaw the relocation of Native American tribes from lands promised in previous treaties.

The "Trail of Tears," as it has become known, resulted in the deaths of more than 4,000 Cherokee alone. Choctaw, Chickasaw, Muscogee, Seminole, and other tribes were also forcibly removed.

The reasoning behind changing Hamilton's status is less clear.

As a founding father of the US and the first secretary of the treasury, Hamilton today is hardly the controversial figure Jackson is. In addition, Hamilton is the architect of the early American financial system, having established a national bank, a system of tariffs, and friendly trade relations with Britain while assuming states' debts to solidify the nascent union.

For its part, the Treasury has posted a FAQ about the new $10 bill. One of the questions addressed: Why the $10 bill and not the $20 bill?

The answer:

"A number of interesting currency ideas exists. Currency is redesigned to stay ahead of counterfeiting. The ACD Steering Committee recommended a redesign of the $10 note next. The ACD will make its next recommendation based on current and potential security threats to currency notes."

The debate over who should replace Hamilton is far from settled. US Institute of Peace president Nancy Lindborg invited people to sound off on Twitter with the hashtag #TheNewTen.

Here's the full video of Lew's announcement via YouTube: