The short answer is that all stockings used to be knitted flat and the seam was added to hold the stocking together. Seamed stockings are the perfect finishing touch to a 1950s style outfit, after all, nobody would be seen without a seamed nylon clad leg during that decade.

What are stockings with the line up the back called?

Hold-ups or stay-ups (in the United States also referred to as thigh-high stockings or simply thigh highs) are stockings with an elasticized band at the top, designed to hold the stockings up when worn, without the use of a garter belt or garters (British English: suspenders).

When did seamless stockings become popular?

The 1950's introduced the seamless stocking. Because it appeared as if the woman was bare-legged, it was first thought to be unladylike, but soon surpassed the seamed stocking in popularity.