Thompson Grapes and raisins aren’t truly seedless. They have small, soft undeveloped seeds in them that go unnoticed.
A Scottish immigrant to California named William Thompson introduced seedless grapes in 1876 specially for use in raisins. At the time, the variety was called Lady deCoverly, but it was renamed after him. Most seedless grape varieties in the United States today derive from crosses between Thomson Seedless and Black Monukka.