Written description issues during prosecution may occurs when an applicant attempts to convert a specific numerical value disclosed in the specification into a broader, open-ended limitation through a claim amendment. The amendment may appear modest. The applicant may believe that a person of ordinary skill would readily understand that if a specification discloses a change of 500 units, then changes of at least 500 units are implicitly contemplated as well. However, written description law focuses on possession, not plausibility. The question is not whether the broader limitation would have been obvious to a skilled artisan or an understandable variation by the skilled artisan. The question is whether the original disclosure reasonably conveyed possession of the broader subject matter.

MPEP § 2163 explains that the written description requirement is satisfied only when the disclosure reasonably conveys to those skilled in the art that the inventor had possession of the claimed invention as of the filing date. This distinction becomes particularly important when dealing with numerical values, ranges, and amendments that attempt to transform a specific disclosed value or set of values into an open-ended boundary.

A recent PTAB decision illustrates the issue particularly well. In Ex parte Tanner, Appeal 2026-000135 (PTAB June 16, 2026), the application involved methods for producing treated sintered compositions used in cathode materials. The written description issue arose from an amendment directed to a cooling step.

The appealed claim required “cooling the sintered composition from the sintering temperature to a temperature at least 700°C less than the sintering temperature.” The applicant relied primarily on an example in the specification describing a process in which material was sintered at 1050°C and then cooled. The specification stated that “[t]he temperature of the LCO ribbon then drops by approximately 700°C in just 3 min.” The applicant argued that this example constituted an actual and constructive reduction to practice sufficient to support the amended claim language.

The Board disagreed. After reviewing the disclosure, the Board concluded that the example described a temperature decrease of approximately 700°C, but did not demonstrate possession of cooling by more than 700°C. As the Board explained: “This language does not indicate that the temperature could or should be dropped by more than 700°C, as required by the claim language (‘at least 700°C’).” The Board therefore concluded that the example was insufficient to establish possession of the full scope of the amended claim.

According to this panel, the claim was broad enough to include temperature drops of more than 700°C, yet the specification did not disclose that the inventor possessed that concept. The Board’s reasoning highlights an important principle where a disclosure of a specific value is not necessarily a disclosure of all values greater than that value. The amendment changed the nature of the limitation. The specification described a temperature drop of approximately 700°C. The amended claim covered any temperature drop of 700°C or more. Those Board found that those are different inventions for written description purposes given the disclosure in the specification.

The Board further noted another problem created by the amendment. The claims permitted sintering temperatures as low as 500°C. If the material were cooled to a temperature at least 700°C below a 500°C sintering temperature, the resulting temperature would necessarily be below -200°C. The Board observed that nothing in the specification suggested cooling below room temperature, much less cooling to such extreme temperatures. As a result, the disclosure did not reasonably convey possession of the full scope of the amended limitation.

The decision serves as a useful reminder that written description analysis is often driven by the outer boundaries of claim scope. Practitioners frequently focus on whether there is support for the embodiment they have in mind. Examiners and the Board frequently focus on whether the disclosure supports other embodiments encompassed by the claim language. When a claim is drafted or amended using open-ended language such as “at least,” “greater than,” “less than,” or “no more than,” the scope often extends well beyond the specific examples disclosed in the specification. If the disclosure does not provide some indication that those additional embodiments were contemplated, a written description rejection may follow.

The case also demonstrates why possession arguments based solely on examples can be difficult. Examples often establish possession of exactly what they describe. They do not automatically establish possession of broader numerical boundaries. An example stating that a temperature decreases by approximately 500°C may support a claim requiring a decrease of 500°C. It may even support a reasonably supported range surrounding that value depending on the disclosure. But it does not necessarily support a later amendment requiring a decrease of at least 500°C, because that language encompasses every larger decrease as well.

The outcome might have been different with slightly different drafting. Suppose the specification had stated that the temperature may decrease by approximately 700°C or more before the subsequent treatment step. Suppose the specification had disclosed that cooling may occur to room temperature or to lower temperatures depending on process requirements. Suppose the specification had described temperature reductions of at least 500°C, at least 600°C, at least 700°C, or greater. Any of those disclosures would have provided stronger evidence that the inventors possessed an open-ended lower boundary rather than merely a single example involving a 700°C decrease.

The lesson for patent drafters is straightforward. Whenever numerical process parameters are described, consider whether future prosecution might benefit from open-ended claim language, in addition to specified range examples. The lesson for prosecutors is equally important. Before amending a claim to introduce language such as “at least,” “greater than,” or “no less than,” carefully examine whether the original disclosure demonstrates possession of the entire expanded scope. Do not assume that a disclosed value automatically provides support for every larger value. Consider whether the specification affirmatively conveys that the inventor contemplated the broader boundary. If the answer is uncertain, consider whether other portions of the disclosure provide support before making the amendment.

When dealing with numerical limitations, the difference between a specific disclosed value and an open-ended claim boundary may appear small. For written description purposes, however, that difference can determine whether a claim survives or falls.