A “folder” is a piece intended to present and protect other pieces that are contained within it. The usual appearance of a folder is, in most cases, similar to that of the bi-fold, but with the addition of a pocket or pockets inside, precisely designed to contain other pieces. That is, a folder generally consists of a cover, a back page with the contact information, and two internal sides, but may be of other types (such as a tri-fold, gate-fold, etc.).
The difference of a folder as compared to other pieces consists primarily in its objective: whereas a bi-fold or a tri-fold is assumed to communicate a message, a folder only contains the pieces bearing the message; a folder is simply a way to protect and give some initial presentation. In many folders, the two inner sides contain little or no information (in the latter case, the printing on one of the sides is avoided in order to reduce the print costs of the piece). The main advantage of folder is a wide range of its possible modifications: varying the inserts, contained inside, one can set it as an institutional piece, a sales piece, a piece for budgets, for internal communication, etc. with a myriad of different uses.
The most common size is 18″ x 12″ or Super DIN A3 (45cm wide x 33cm high), since this size allows for storing LTR or A4 size sheets of paper or inserts (sheets for use in a folder) inside the pocket or pockets.
In most cases they incorporate a pocket or pockets for sheets (called inserts, which can be sometimes staggered) or business cards.