Why are Levi jeans called 501?

Related questions

1. What are Levi 501 jeans?
2. Does Levi's make Wrangler jeans?
3. What do Levis numbers mean?
4. Does Levi's use real leather?
5. What is the difference between Levi's 505 and 514 jeans?
6. Is Levi Strauss the same as Levis?
7. Are Womens Levis true to size?
8. When did Levis stop making jeans in USA?
9. Are Lee Jeans Made in USA?
10. Are Lee and Levi's the same company?

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