Barcode symbology
The set of rules deciding how data is turned into bars and spaces in a given kind of barcode.
A symbology defines the character set, how each character is encoded as widths, the start and stop patterns, any check digit, and how wide the quiet zone must be. Linear (1D) symbologies such as Code 128, Code 39, UPC and EAN encode data in parallel bars; matrix (2D) symbologies such as QR and Data Matrix use a grid and hold far more. The choice depends on what is encoded and where: a supermarket till wants EAN, a parcel may need 2D. Finding the bars does a reader little good if it cannot decode the symbology — so modern scanners support dozens and guess automatically which one they see.