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The UPC/EAN/JAN standards include specifications for some of the bars in the symbol to extend below its main body. These bars are known as guard bars, and they serve as reference points for scanning devices. In a standard full-sized symbol, the main body of the barcode is 0.9 inches tall, and the guard bars extend another 0.065 inches.

If your UPC/EAN/JAN barcodes are of a different height than the standard, you might need to adjust the length of the guard bars to keep them correctly proportioned relative to the rest of the barcode and to the human readable characters.