standard. http://en.wikipedia.org/wiki/Optical_comparator even more widely used. Quote: http://en.wikipedia.org/wiki/Screw_t...tandardization During this era, in continental Europe, the British and American threadforms were well known, but also various metric thread standards were evolving, which usually employed 60° profiles. Some of these evolved into national or quasi-national standards. They were mostly unified in 1898 by