Set bars and spaces, readable in horizontal direction - barcode are vertically "redundand". This barcode category is older, but still used at most.

Set bars and spaces, readable in horizontal direction, but in difference to linear barcodes stacked barcodes is set of linear barcodes in vertical order. This barcode type is still vertical "redundant".
For example PDF417 is often referred to as a 2D symbology. However, it is not a true matrix code. It is a stacked linear code, consisting of multiple rows of short linear codewords. It allows bars and spaces to have up to 6 different widths and as such it can be adversely affected by binarization at lower samples per module.

Matrix (2D) codes are created as matrix square, circle or hexagonal symbols.
They use a regular grid of possible cell positions, and the presence or absence of a cell in a grid location encodes the data.
Matrix barcodes have higher capacity as linear barcodes, have implemented powerfull error correction.

Barcode symbologies used in post office. More country have own specifications.

• Some symbologies can be derived from Code39 or Code93 (Danish, French, Dutch postal symbologies)
• use own encoding system ( Telepen, CPC )
• Some use 4-state symbology ( BPO4State, Royal Mail, Singapoore postal, Australia post, Japan Post, Korean, Deutsche 4 state, IntelligentMail...), information isn't encoded into sequence lines and bars with different width, but encoded using lines in four different height ( if we separate bar in height to three same parts ):
  
  

  Basic elements of Postal barcodes:

  
  
  • T - Tracker
  • D - Tracker, Descender
  • A - Tracker, Ascender
  • H - Tracker, Ascender, Descender
• ... or use 2-state encoding - lines are full in height or bottom half ( PostNet ) .