Code 39 Extended was developed to provide a means of encoding additional characters that are not part of the Code 39 character set (for example, lowercase characters and additional symbols), and is essentially a superset of Code 39. Extended characters are encoded by using a pair of normal Code 39 characters. For example, a lowercase "a" is encoded by the character pair "+A". Code 39 Extended supports the full ASCII character set.

The ratio of wide to narrow bar width can be in the range of 1.8 to 3.4. The Barcode XpressCode 39 style uses a 2:1 wide:narrow bar width ratio.

Code 39 Extended requires a quiet zone, which should be at least the width of 10 narrow bars. This means that if you set the MakeBarcodeBarSize property to 3 pixels (narrow bar size), you should set the MakeBarcodeBWidth property to 30 pixels. Since Code 39 is a self-checking barcode, it normally does not require a checksum. However, when necessary, an optional modulo 43 checksum may be used.

By default, Barcode Xpress will not write optional checksums. If a checksum is desired for Code 39, set the BarcodeAppendCheckSum property to True and Barcode Xpress will calculate and write the checksum.