A DEC file contains a card deck for Magic: The Gathering Online (MTGO), an online card-playing game. It stores a collection of cards that make up a playable deck, which includes the number of cards and the names of each card in the deck. DEC files may also contain the names of sideboard cards and notes.

DEC file open in TJ Soft Decked Builder 1.25 DEC files are saved in plain text, with each line beginning with the number of cards followed by the name of the card. Some lines may also begin with SB:, which indicates that they are sideboard cards, or //, which indicates a note. Since the files are saved in plain text, you can easily create, modify, and share playable decks. You can open and view DEC files with any text editor, such as Notepad, WordPad, and TextEdit. However, you should only modify and save DEC files with plain text editors to avoid saving extra formatting information, which cannot be read. Also, you may have to change the file extension of a deck saved using a text editor from .txt to .dec. NOTE: Playable decks may also be saved as .DEK files.