Levi's introduced the 501 model number in 1890.
The number appeared after Levi Strauss & Co.'s original rivet patent entered the public domain in 1890. With competitors free to make similar riveted denim garments, the company assigned lot numbers to its products, using 501 for its famous copper-riveted waist overalls. The number therefore began as a product or lot designation rather than a reference to the year 501, a person or a particular measurement.
The 501 name is connected to the earliest development of modern blue jeans. Levi Strauss and tailor Jacob W. Davis received a U.S. patent for strengthening pocket openings with rivets on May 20, 1873. The company later refined the workwear design, and the 501 evolved into a long-running clothing icon.
A frequent mistake is answering 1873 because that was the patent year, or 1890 because it marks the patent's expiration without recognizing the model-number connection. The exact reason Levi's chose “501” is unknown: company records were destroyed in the 1906 San Francisco earthquake and fire.