Interleaved 2 of 5 is a numeric-only barcode widely used in warehouse and industrial applications. The data must consist of an even number of digits.
Each data character is composed of 5 elements, either 5 bars or 5 spaces. Of these 5 elements, two are wide and three are narrow. Adjacent characters are interleaved, alternating the spaces from one character with the bars of the other.
The Interleaved 2 of 5 specification does not include a checksum requirement, but almost all applications of the code call for a checksum. Because of the structure of I2of5, is is possible for a scanner to pick up only a portion of the barcode and mis-interpret it as a complete code. This makes use of a checksum imperative (see ITF-14 for an example).