What Is Written Description?

Written description is the legal requirement that a patent specification must demonstrate that the inventor was "in possession" of the claimed invention at the time of filing. It is not about whether someone could build the invention from the specification (that is enablement) — it is about whether the specification shows the inventor actually had it.

The distinction is subtle but consequential. A specification can teach someone how to make something without proving the inventor had already conceived of it in the specific form being claimed. Written description catches this gap.

Why Written Description Matters

Written description rejections arise most frequently in two situations:

New matter. When an applicant amends claims during prosecution to add features or language that were not present in the original specification, the examiner may issue a written description rejection on the ground that the new claim language introduces "new matter" — subject matter the inventor did not possess at the time of filing. You cannot add to the specification after filing.

Genus claims supported by limited species. In biotechnology, pharmaceuticals, and chemistry, applicants often claim a broad genus (a family of related compounds, sequences, or structures) but disclose only a few specific species. The examiner may argue that the specification does not demonstrate possession of the full genus — only the specific species tested. The written description requirement demands that the specification convey to a skilled person that the inventor had conceived of the full scope being claimed.

The Legal Standard

In the US, the Federal Circuit's decision in Ariad Pharmaceuticals v. Eli Lilly (2010) confirmed that written description is a separate requirement from enablement under 35 U.S.C. § 112(a). The test is whether the specification "reasonably conveys to those skilled in the art that the inventor had possession of the claimed subject matter as of the filing date."

Possession can be demonstrated through structural descriptions (describing the physical or chemical structure), functional descriptions coupled with a known correlation between structure and function, or a combination of both. The level of detail required depends on the art — mechanical inventions with predictable structure-function relationships require less than biotechnology inventions where the relationship is unpredictable.

Written Description vs Enablement

A well-drafted specification satisfies both simultaneously. But they fail independently — a specification can pass enablement and fail written description, or vice versa.

Practical Guidance for Inventors

Describe every variation you can envision at the time of filing. If you intend to claim a range of materials, describe representative examples across that range. If you intend to claim a genus of compounds, synthesise and characterise enough species to demonstrate possession of the genus.

Do not rely on amendments to add scope later. The original specification sets the ceiling for what can ever be claimed. Features not described in the original filing cannot be added through amendment without triggering a new matter rejection.

Keep detailed invention records. Dated lab notebooks, experimental data, and prototype documentation support the argument that you possessed the invention at the time of filing — particularly valuable if written description is challenged in litigation or post-grant proceedings.

Sources

1. 35 U.S.C. § 112(a) — Written Description — US statutory requirement for written description in patent specifications
2. MPEP § 2163 — Written Description Requirement — USPTO guidance on possession and written description analysis
3. EPC Article 83 — Disclosure — European disclosure requirement overlapping with US written description