You are using some wrong terminology.
The correct term is Visa Exempt Entry.
This is from the US Embassy site:
Visa Exempt Entry
U.S. citizens carrying a tourist passport and in possession of an onward or return airline ticket do not require a visa to enter Thailand. The passport must have at least six months validity remaining to be allowed entry. Upon entry, Thai immigration officials will place an immigration stamp in the passport permitting a 30-day stay in Thailand if arriving by air or land. This time limit may subsequently be extended for an additional 30 days by paying a 1,900 baht fee to the Thai Immigration Bureau office.