A character in Code 39 fonts is mapped to itself, with exception of space. Since space is reserved for many other purposes, it must remain blank. To print a barcode character that will be scanned as space, use underscore (_) or equal sign (=).

To print start/stop characters, use asterisk (*). You can also use left square bracket [ and right square bracket ] to produce them. The only difference between square brackets and asterisk is that the latter has human readable character * beneath the barcode in a human readable font. In non-human-readable fonts, they print identical patterns.