What are special technical features in unity of invention?

This page is an FAQ based on guidance from the Manual of Patent Examining Procedure. It is provided as guidance, with links to the ground truth sources. This is information only: it is not legal advice.

Special technical features are a crucial concept in determining unity of invention. According to MPEP 1893.03(d):

“The expression special technical features is defined as meaning those technical features that define the contribution which each claimed invention, considered as a whole, makes over the prior art.”

These features are used to establish a technical relationship among the inventions. For example:

• A key defined by certain claimed structural characteristics corresponding to the claimed features of a lock would be a special technical feature.
• The technical relationship must involve at least one common or corresponding special technical feature among the inventions.

Examiners use these special technical features to determine whether multiple inventions form a single general inventive concept, thus meeting the unity of invention requirement.