There's nothing like a bit of history to illuminate the why of things. Like with inner envelopes—what are they and why do they exist?
An inner envelope is just what its name suggests—it goes inside the main, outer envelope. It's also the piece that addresses its recipient(s) by name, is lined and contains all the pieces of the wedding invitation suite. To trace the origins of this two envelope system, look no further than Downton Abbey, a.k.a. the era of the hand-written, hand-delivered note. Consider an errand Thomas might have been tasked with—traveling by foot, on dirt roads and through variable weather conditions, to deliver Lord Grantham's correspondence ASAP! You can see how an exterior envelope would get dirty through all that. But take heart! Upon delivery, the outer envelope would be discarded and the inner envelope, pristine and closed via wax seal, would be presented (on a silver tray, of course!) to the important Lord or Lady on the other end.
It's no surprise that we love the inner envelope over here, and the photo above is of one of our recent favorites, complete with a liner that's reminiscent of a hardcover classic! It's an elegant nod to tradition and presents your wedding invitation as it should be received—completely unmarred. It's also handy for explicitly stating the invitees, which can be a subtle way to indicate that no, you cannot bring your neighbor's ex-wife's cousin as your guest; his name is not on the inner envelope!