Canvas coding query

Google-Fu not working too well on this one
I am going though my stash of cotton duck and I see these coloured threads along the selvage edge, it seems as if the heavier the fabric the more coloured threads but I can't find any information on the code involved
Does anybody know anything about it??
I am going though my stash of cotton duck and I see these coloured threads along the selvage edge, it seems as if the heavier the fabric the more coloured threads but I can't find any information on the code involved
Does anybody know anything about it??