More traditional and formal invitations tend to use an inner and outer envelope. On the inner envelope, you’d write the name of the person or persons invited, such as “Ms. Keats and guest.” More modern invitations—or couples looking for something more streamlined—often forgo the inner envelope (though in some regions of the country (i.e. the South), an inner envelope still seems to be a “must”). In most cases, it really depends on what you and your family prefer. Our rectangular Bella envelopes, which we custom made just for us, have beautifully snug inner and outer envelopes.
If lining your envelopes, we do recommend using an inner and outer envelope combination. If you only use an outer envelope, and that envelope is lined, guests will tend to tear that envelope open and miss the perfect burst of color that an envelope lining gives a set.