The use of two envelopes for wedding invitations dates back to the horse-and-buggy days before the dawn of the 20th Century. The postal service was not very reliable at that time and non-existent in certain areas. The only way to guarantee that a wedding invitation was delivered was to hire a courier or use a family servant to hand deliver the invitation via horseback. In order to keep the invitation in the most pristine condition possible during the oftentimes rugged journey, two envelopes were utilized.
Once the invitation arrived to its intended destination, it was actually delivered to the household butler or servant who then removed the outer envelope before delivering it to the head of the household. Even though the postal service is now the most reliable way to deliver invitations today, the use of two envelopes is touch that adds a sense of tradition to your wedding stationery ensemble.
If your invitations come with double envelopes (which most standard packages do) here are the rules for using both envelopes:
The inner envelopes are addressed "Mr. and Mrs. Johnson" (first names are omitted). If other family members are included, write out their names on the outer envelope as well: "Mr. and Mrs. Johnson, Richard and Susan". There should not be a street address on the inner envelope. Once properly addressed, the inner envelope is placed inside the outer envelope which is then sealed addressed and processed. The way to tell which envelope is the inner envelope and which one is the outer envelope is that the outer enveloped will have a gummed flap and the inner envelopes will not. Also, the inner envelopes typically have a colored liner.