Interleaved 2 of 5 is a high-density, self-checking, continuous numeric symbology.
It gets the name "Interleaved" because two numeric digits are interleaved together, with the bars representing one digit and the spaces representing a second digit. It is one of the most popular symbologies used by the shipping and warehouse industries for identification. The binary level barcode is suitable for printing on rough-surfaced materials, such as corrugated boxes or cardboard.
The Interleaved 2 of 5 symbology is also referred to as USS ITF 2/5, ITF and I-2/5.

The Distribution Symbology Study Group has recommended 2 of 5 as the standard for numeric labeling of corrugated shipping containers. It has been adopted by the Uniform Product Code Council as the standard symbology for outer shipping containers in the grocery industry. It is also used in the medical and automotive fields.

(The above sample is an Interleaved 2 of 5 barcode encoding a 12 digit number)

Every Interleaved 2 of 5 character actually encodes two digits; one in the bars, and one in the spaces. There are five bars, two of which are wide and three of which are narrow. Similarly, there are five spaces in each character, two of which are wide and three of which are narrow. Each digit has its own unique 2 out of 5 arrangement.

Unfortunately, a partial scan has a high probability of decoding as a valid, but shorter Interleaved 2 of 5 symbol. This is due to the simple nature of the start and stop patterns. Because of this partial scan problem, Interleaved 2 of 5 is best used in a fixed-length application, with all reading equipment programmed to accept messages only of the correct length. 14 or 16 digits are generally used for the distribution industry.

Bearer bars are used to help prevent the partial scan from being decoded as a valid Interleaved 2 of 5 symbol. They must tough the top and bottom of all of the data bars. Minimum width of bearer bars should be three times of the narrow bar width. A partial scan of the symbol will now penetrate one or more bearer bars, and the resulting apparent arrangement of wide and narrow elements will not bear any resemblance to a start or stop character ? to prevent an erroneous decode.

An item to note with respect to Interleaved 2 of 5 is that the number of digits in the barcode must be even. This is due to interleaving (one odd digit for every even digit). As such, sometimes a zero digit must be added to the beginning of the barcode to make to total digits an even number. It is recommended to use a check digit and Modulo 10 is generally used for the checksum.

Interleaved 2 of 5 is commonly used for ITF-14, EAN-14, SSC-14, DUN14 and USPS special services such as return receipts and delivery confirmation. These implementations usually require a MOD 10 check digit. The Interleaved 2 of 5 MOD 10 can be calculated by automation tools or manually.
Manual calculation is obtained by adding up the odd numbers in the code and multiplying them by 3 and then add up the even numbers. The check digit is obtained by determining the number that when added to the sum will produce a multiple of 10. For example, the check digit for 123456789 is 5 because (3*(1+3+5+7+9))+(2+4+6+8) = 95 and 95 + 5 =100. When calculating check digits, the data to encode should be an odd number of digits because the check digit is interleaved with the last digit.

The Interleaved 2 of 5 barcode may also be used as for German Post Identcode and Leitcode symbols. Identcode and Leitcode require a checksum to be calculated. The calculation is similar to USS Interleaved 2 of 5 MOD 10 except that even numbers are multiplied by 9 and odd numbers by 4. For example, the check digit for 12345678901 would be "6" because (4*(1+3+5+7+9+1))+(9*(2+4+6+8+0)=284 and 284 + 6 = 290.