As seen in the image to the right, the Australian Post 4-State Code contains four bar types, defined as follows:
- Tracker. Designation “T”, bar state value 3 — This smallest bar only occupies the middle of the barcode
- Descender, Tracker. Designation “D”, bar state value 2 — This bar begins in the middle of the code and descends to the bottom.
- Ascender, Tracker. Designation “A”, bar state value 1 — This bar begins in the middle of the code and ascends to the top.
- Tracker, Ascender, Descender. Designation “H”, bar stat value 0 — This bar spans the entire height of the barcode and includes all three previous states.
The Australian Post 4-State Code contains several format elements, including:
- Start Bars — The first two bars of this code are “start bars,” which identify the orientation of the symbol. Their values are always 1 and 3.
- Format Control Code Field — The next three bars identify the specific format of the barcode, such as Customer Barcode 2, Customer Barcode 3, or Reply Paid Barcode.
- Sorting Code Field — Bars 7 through 22 encode data that identifies the delivery address using an eight-digit Delivery Point Identifier (DPID).
- Customer Identification Field — Used only for Customer Barcode 2 and 3 formats, this element must be 16 or 31 bars long.
- Filler Bars — If the customer identification field isn’t naturally 16 or 31 bars long, filler bars are used to make up the difference. These bars are always tracker bars with a value of 3.
- Reed-Solomon Error Correction Bars — 12 bars long, this error correction component is located just before the stop bars.
- Stop Bars — The final 2 bars of the code always have values of 1 and 3.
It’s also worth noting that the Australian Post 4-State Code requires a “quiet zone” beside, above, and below each barcode. Spaces on either side must be at least 6 mm and those above and below must be at least 2 mm. This barcode supports the encoding of numeric digits 0-9, uppercase and lowercase letters A-Z, number signs (#), and spaces.
Depending on the barcode format type, the Australian Post 4-State Code has differing bar counts along with minimum and maximum lengths. For example, a Standard Customer Barcode is 37 bars long with a minimum length of 37.0 mm and a maximum length of 42.2 mm. Customer Barcode 3, meanwhile, comes in at 67 bars long with a minimum length of 67.5 mm and a maximum length of 76.8 mm. The Reply Paid barcode is 37 bars long but has no minimum or maximum lengths.