What does Red Lines mean?
Denim jeans made before 1986, which have a red line selvedge.
Heddels explains Red Lines
Levi’s red line jeans are easily identified by the red line running down the selvedge on the inside of the pant legs. In the early twentieth century, Cone Mills, the manufacturer of Levi’s denim, added the colored lines to their denim to help differentiate between the different grades of the fabric.
Red line was considered the top grade and collectors argue that jeans made of this type of denim are heavier and superior to modern jeans.
Additional Resources
Close up shot of red line selvedge:
Source: thefedoralounge.com