Interleaved 2 of 5 (ITF) is a continuous two-width barcode symbology encoding digits. It is used commercially on 135 film, for ITF-14 barcodes, and on cartons of some products, while the products inside are labeled with UPC or EAN. ITF encodes pairs of digits; the first digit is encoded in the five bars (or black lines), while the second digit is encoded in the five spaces (or white lines) interleaved with them. Two out of every five bars or spaces are wide (hence exactly 2 of 5). The digits are encoded to symbols as follows: where "n" is a narrow line (bar or space) and "W" a wide line (2.0 to 3.0 times the width of a narrow line). The wide lines form a two-out-of-five code with consecutive values of 1, 2, 4, 7, and 0, where the code 0 has a value of 11. This is similar to the POSTNET bar code. Before the actual pairs there is a start code consisting of nnnn (narrow bar - narrow space - narrow bar - narrow space), and after all symbols there is the stop code consisting of Wnn (Wide bar-narrow space-narrow bar). Because digits are encoded by pairs, only an even number of digits can be encoded. Typically an odd number of digits is encoded by adding a "0" as first digit, but sometimes an odd number of digits is encoded by using five narrow spaces in the last digit. A checksum can be added as last digit, which is calculated in the same way as UPC checksums. There are specific constraints on the height and width of the bars and the width of the "quiet zones", the blank areas before the start and after the stop symbol. ITF is often used for marking product ID numbers or other codes, of various lengths, on item cartons and multi-unit cases. One specific instance of this, standardized by GS1, is the ITF-14 bar code used to mark packages with Global Trade Item Numbers. In these uses, the ITF bar code symbol is usually printed surrounded with a thick black rectangular border called the Bearer Bar. The Bearer Bar surrounds the entire symbol and the quiet zones, abutting the ends of the bars. The purpose of a Bearer Bar is to equalize the pressure exerted by the printing plate over the entire surface of the symbol, and to enhance reading reliability by helping