Guide
How to Read Patent Classification Codes
A patent classification code reads from left to right, from broad to specific. Take H04L 9/32: the letter names a section, the digits a class, the next letter a subclass, and the number pair a group. Five levels, one line. This guide decodes each level, using the same code in IPC and CPC.
The five levels of H04L 9/32
| Level | Code | Meaning |
|---|---|---|
| Section | H | Electricity One letter, A to H. The CPC adds section Y for cross-cutting technologies. |
| Class | H04 | Electric communication technique Section letter plus two digits. |
| Subclass | H04L | Transmission of digital information Class plus one letter. Most searches start at this level. |
| Main group | H04L 9/00 | Cryptographic mechanisms One to three digits before the slash, always /00 after it. |
| Subgroup | H04L 9/32 | Verifying identity or authority Any number after the slash except 00. The most specific level. |
The /00 convention
A number ending in /00 marks a main group, the top entry under a subclass. G06N 20/00 (machine learning) is a main group. Every other number after the slash, such as /32, marks a subgroup inside its main group.
Dots define depth, not numbers
Inside a main group, indentation dots define the hierarchy. A subgroup with two dots nests under the nearest subgroup above it with one dot, regardless of its number. So a higher number does not mean a deeper level; only the dots do. The tree views on this site render that nesting directly, for example under H04L 9/00.
Where you see these codes
- On the front page of every published patent document, in the field labeled (51).
- In every major patent database as a search filter, in IPC or CPC form.
- In statistics and landscape reports, usually aggregated at class or subclass level, as in our global filing statistics.
One patent, several codes
A single patent usually carries several classification codes. The first classification names the core invention; additional codes cover its other technical aspects. A neural-network chip, for example, can sit in G06N for the model and in H10D for the semiconductor structure. Do not confuse any of this with kind codes such as A1 or B1: those mark the publication stage of a document, not its technology.
Finding the right code
Browsing works well when you know the field: start at a section page and descend. When you only have a description of the technology, let the classifier suggest codes: