A tang stamp is the maker's mark pressed into the tang of a blade, visible on a folder when it is open. It carries the brand, usually a country of origin, and on some American brands a code that dates the knife. To a collector it is the closest thing a production knife has to a birth certificate.
At minimum a brand name. Often a country of manufacture, a pattern number, and for some makers a dating code. One example is Case: from 1970 it placed ten dots beneath the CASE XX USA stamp and removed one each year, so nine dots is 1971 and one dot is 1979, with a different system for the 1980s.
Buck's marks changed by era rather than following one scheme, with a formal date-code symbol system introduced in 1986.
Two visually identical knives can be worth very different amounts depending on when they were made, and the stamp is the evidence. It is the first thing to check against any listing that claims an age.
A worn or illegible stamp reduces value, because it removes the identification a collector relies on.
Only the eras and milestones a manufacturer documents itself are reliable. Charts circulating online that claim precise years the maker has never published are folklore, and we will not reproduce them.
See Case date codes and Buck date codes for what is actually sourced.