As the best-selling American whiskey in the world, Jack Daniel’s is a bar-cart staple. And the easy-to-recognize, classic logo design has remained essentially the same since the founder created it himself at the end of the 19th century.

While the label is otherwise straightforward and simple, there’s one vague element of the famous logo that has led to debate among aficionados. Centered near the top of the black and white label is the curious phrase “Old No. 7,” leading to a variety of theories about what Daniel meant with the inclusion of the wording.

“The mystery of Old No. 7 is one that has surrounded Jack Daniel’s for over 150 years,” says Svend Jansen, the brand’s public relations director.“It’s the No. 1 question asked by visitors to the Jack Daniel’s Distillery.”

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Some argue that seven was the founder’s lucky number, while others insist it was the seventh batch or recipe attempt that proved successful. With Daniel’s reputation as a ladies’ man, others claim that the number honored his seventh girlfriend.

Trademark filings from 1940 indicate that the Daniel family claimed the whiskey was made the same way for seven generations, but no distinction was made regarding the reason for the label.

Peter Krass, author of “Blood & Whisky: The Life and Times of Jack Daniel,” believes he found the real reason that the bottles are labeled Old No. 7 in a newspaper from 1877. While researching vintage publications for his book, Krass found an article documenting a change in the tax districting system regulating local distillers. At the time, distillers in Lynchburg were categorized in tax district No. 4, and the Jack Daniel’s Distillery was issued registration No. 7. Labeling requirements dictated that the distiller’s registration number be placed on bottles.

In subsequent years, the government amended the structure, combining Lynchburg’s district 4 with Nashville’s district 5. As a result of the shift, the registration number for Jack Daniel’s was changed to 16.

Daniel’s distillery was the only one who’s number was changed during the shuffle. Thus, according to Krass, Daniel made the decision to adorn his jugs and barrels with Old No. 7 in an act of defiance against the revenue department and to remind customers about his brand.

It’s anyone’s guess which story rings true, as Daniel apparently took his reasoning with him when he died in October, 1911. This air of mystery adds another element to the label’s allure — a fact that has not gone unnoticed by the brand. “The truth is, we genuinely don’t know the significance of the No. 7, but this single digit occupies a unique place in Jack Daniel’s history,” Jansen says.