The other day I helped my cousin assemble her wedding invitations and the question of whether the liner should go in the inner or outer envelope came up.
Contrary to what she thought, the liner should actually go inside the inner envelope.
But, if the inner envelope and its contents are considered the "real" thing, why have an outer envelope at all? The answer is that you don't really need one. The practice of enclosing wedding invitations within an extra envelope began back in the "pony express" days, when mail was delivered by horse, to protect the invitation from mud and dust. Likewise, the little papers that are often placed in between the different components of the invitation itself were originally used to protect each piece from getting smudged with ink. Current technology eliminates the need for these items, although their use will give your invitations a nice sense of tradition and ceremony, denoting the importance of the contents!