Last weekend, Mayor Vince Gray celebrated the birth of a new sibling: Ankara, Turkey, which became Washington’s 12th sister city. Which made us curious to know: How many sister cities do we have, anyway? And what benefits does sisterhood convey?

The first question is easy: D.C. is “related” to Beijing, Seoul, Brussels, Rome, Paris, Accra, Pretoria, Dakar, Bangkok, Sunderland, Athens, and now Ankara. The first sister city agreement, with Bangkok, was signed in 1964. But the District has been picky.

“We have many, many requests to become a Sister City with countries all over the world, so we limit our relationships to capital cities,” says mayoral spokesman Robert Marus. “Sunderland, England—-the birthplace of George Washington—-is our only exception.” (UPDATE, 4:50 p.m. – Marus may have meant that Sunderland was the birthplace of Washington’s ancestors).

Although that policy excludes some pretty rad places—-and we’ve managed to leave out the entire Western Hemisphere anyway—-you can see the logic. Seats of government share quite a few challenges in common. Plus, you’ve got to draw the line somewhere, and sticking with capital cities is as good a form of quality control as any (less famous cities have to search around for someone to sister up with, often through Sister City International, which functions as something of a metropolitan sibling finder).

The second question is a little harder to answer. Some of the sister city agreements stipulate or lead to real cultural or business exchange, like delegations of students going back and forth, glassmakers meeting for joint shows, and tourism cross-promotion with Destination DC. But most are written in the vaguest of terms, simply encouraging the two cities to “share ideas.” Which I’d hope the District would be doing anyway.

At least the mayors get a photo op out of the deal.