Code 39 - also called Code 3 of 9 or USS Code 39 - is one of the oldest barcodes still in use, and one of the easiest to read. It holds capital letters, digits and a few symbols, and you'll find it on industrial parts, ID badges and warehouse shelves, and in the automotive and defense industries.

What it holds
Characters	Capital letters, digits, the space and - . $ / + %. Lowercase letters are turned into capitals.
Length	Up to 74 characters.
Check digit	Optional: a Mod 43 character at the end, which you turn on with Check digit.
Modes	Standard, or Extended - also called Full ASCII - which takes lowercase letters and every other symbol on the keyboard as well. Each of those uses two characters of the barcode, so an Extended value of lowercase letters fits only 37. The scanner has to be set to read Extended.
Encoding	Standard or HIBC, for the health industry: see GS1 and HIBC data. HIBC is always a Standard Code 39, whatever the Mode.
Readable text	The value, under or over the bars. The asterisks that start and end every Code 39 are added for you and aren't printed - don't type them.

Check digit. Barcode and text adds the check character to the bars and prints it at the end of the value; Barcode only adds it to the bars alone. Many Code 39 readers ignore it, so use it only if yours expects one. With HIBC the setting isn't shown: HIBC always has a check character of its own.

Characters you can't type. In Extended, a tilde followed by three digits is the character with that code - ~009 is a tab - and ~~ is a tilde.

The rest of the settings - text position, quiet zone, color - are in the Barcode section.

What goes wrong
Invalid characters in input data.	The value has a character Code 39 can't hold: an accented letter, or, in Standard, a symbol such as * or #. Switch to Extended for symbols.
Input data too long.	More than 74 characters - or, in Extended, too many lowercase letters and symbols.

Code 39 is wide for what it holds. For a long value, Code 128 takes the same characters in much less space.