I assume you are talking about this, which mentions a 6 year limit: https://www.law.cornell.edu/uscode/text/35/286 https://www.law.cornell.edu/uscode/text/35/2

I assume you are talking about this, which mentions a 6 year limit: https://www.law.cornell.edu/uscode/text/35/286 https://www.law.cornell.edu/uscode/text/35/286 But that only limits the time period for which damages may be claimed - it does not forfeit the ability to claim damages altogether, which is what may happen with trademarks; that's a pretty significant difference. Ok, that is an interesting aspect of US patent law I wasn't familiar with. But this doesn't look like a universal property of patents, more likely it's specific to the US?