The Royal Mail barcode contains six key elements:
- A Leading Quiet Zone
- One Start Bar (Always Tracker, Ascender)
- Symbol Characters that Represent Postcodes and DPS
- A Check Digit
- One Stop Bar (Always Tracker, Ascender, Descender)
- A Trailing Quiet Zone
As noted above, the RM4SCC is a 4-state barcode, which means that each bar can be in one of four states, except for the start and stop bars which always follow the same pattern. The four states are:
- Tracker (T) — Bars in this state occupy only the middle third of the barcode’s height.
- Tracker, Descender (D) — Bars in this state occupy the bottom two-thirds of the barcode.
- Tracker , Ascender (A) — Bars in this state occupy the top two-thirds of the barcode.
- Tracker, Ascender, Descender (H) — Bars in this state occupy the entire height of the barcode.
By varying these states across the width of the barcode, it’s possible to encode numbers, capital letters, and parentheses.
Positioning of these barcodes is also critical on Royal Mail packages and letters. The code must be no less than 18 mm and no more than 120 mm from the bottom edge. It must also be no less than 15 mm down from the top edge and no less than 15 mm from each side. Codes can be printed at any right angle — 0, 90, 180, and 270 degrees — and still be accurately read.