Hillary Clinton, who defended her use of a private e-mail server by saying she only wanted to use one device for all her correspondence, actually used at least two devices: her regular mobile phone and an iPad.

That's according to a report from the Associated Press, which reviewed Clinton's e-mails released by the State Department. There were only four messages to see. But it was enough for the AP to conclude that Clinton not only used an iPad to send e-mails — she mixed work and personal e-mails on the tablet, too.

"In reply to a message sent in September 2011 by adviser Huma Abedin to Clinton's personal email account … Clinton mistakenly replied with questions that appear to be about decorations," the AP writes. Clinton quickly wrote Abedin to apologize for the message, which was meant for somebody else. But then Clinton added: "Also, pls let me know if you got a reply from my ipad. I'm not sure replies go thru."

The anecdote undercuts Clinton's argument for having a private, nongovernmental e-mail server. Weeks ago, she said that it seemed "easier to carry one device for my work… instead of two." The iPad revelation suggests that as secretary of state, Clinton sent work e-mail on more than one device. (Previously, Slate wrote about the many devices of Hillary Clinton, including an iPad mini and iPod.)

Clinton's fundamental issue — having to juggle multiple phones — is a frustration she shares with many of us.

I asked Deloitte, which publishes the annual Global Mobile Consumer Survey, how many people in the United States carry more than one phone with them. While their research on this question isn't freely available and doesn't break it down by country, Deloitte's response suggested that 1 in 5 people own or have ready access to multiple smartphones at a time. Four percent have access to three smartphones. And one percent uses five or more.

We're not even counting tablets or other mobile devices at this point. Just smartphones. (These people must be gadget reviewers or rocket scientists or something.)

Here's the detailed data they sent me:

You can't conclude from these numbers that the multiple-smartphone users are all Washingtonians who have their digital lives separated on work BlackBerrys and personal iPhones. They just give you an idea about how common the frustration associated with the constant switching must be.

Not all of us can be secretary of state. But if anything, that underscores how odd it is that Clinton felt compelled to have a separate e-mail server. If so many of us are forced to lug around a work and personal phone, why shouldn't she?

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