“Teaser” copy is a word or phrase printed on the envelope. Often it is the same or very similar to the headline inside your mailer. Meant to be compelling or sparking curiosity, the purpose of a teaser is to get your reader to open the letter.
Everyone likes a good teaser. Seeing one with a window envelope creates a strong curiosity factor- one where your readers have a great desire to see the rest of the envelope contents, one associated with them. Enhancing the teaser, have a second window to your envelope where you can display a photo of the product you are writing about or you, a loyal customer or celebrity making eye contact.
It has been shown that teasers can get a positive response to opening the envelope by 250%. Those are great odds in your favor!