How many of these components of an envelope do you know? 

Most of us use or handle envelopes everyday. But do you know how an envelope is constructed? The envelope you design or choose for your desktop publishing projects is just as important as what goes in it. The size of the piece, type of mailing, budget, and whether or not you'll be using automated equipment to insert the envelope contents affects the style of envelope you can use. You can also choose specific envelope sizes and styles to enhance a personal or business image, invoke a particular action, or create a certain aura. When discussing envelope options with clients and printers, basic knowledge of envelope construction can help you cut costs and choose the best envelope for the project.

Face or Front
The front of the envelope, usually seamless, may have windows that allow the inside contents to show through. The face of the envelope is where the address, postage, and usually the return address appear.

Back
The back of the envelope, typically left blank, is where the flaps meet to form and seal the envelope. 

Flaps
The parts of an envelope that are folded, overlapped, and sealed to enclose the contents are typically rectangular or triangular with rounded, tapered, or pointed corners. The typical envelope consists of two side flaps, a bottom flap, and a top flap. The side flaps are folded in first with the bottom flap folded up. They are sealed where they overlap. The top flap is folded over the side and bottom flaps and sealed after inserting the envelope contents. The choice of envelope flaps not only affects appearance, but is an important consideration when using automated equipment which may require certain sizes and styles of flaps. Some automatic-insertion machines may only support nonstandard flap sizes and shapes

Seams
The style of flaps determines the type of seams -- the edges where the envelope flaps meet and overlap.   

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Folds
The creases formed at the sides, top, and bottom between the face and the back when all the flaps are folded to the back of the envelope are the folds.

Envelope Openings and Closures                
Envelopes have one side left open and unsealed for inserting material. Non-square envelopes are either open end or open side. Open side is the most common, even though most letter mail envelopes appear to open on top. The opening is determined not by orientation of the top flap but by the length of the side where the opening appears. In addition to the style or position of the flap, envelope closures may be with or without adhesive. 

 
Window
Some envelopes have one or more cut-out areas, usually on the face of the envelope, so that a mailing address, return address, or special message shows through from the inside. Windows may be left open or might have a clear or tinted covering. Window envelopes can be custom-designed or purchased with window sizes and positions

The envelope, initially a simple covering, has become a means of communication with many resources thanks to the art of advertising designers and the work of the manufacturers. A combination of just three elements (printing, folding and added pieces) pockets and wallets produced for commercial and advertising envelopes and a whole range of other uses.