scope:

There is no end to the development and improvement of an active standard. This revised edition of B1.1 marks an important step forward in that it embodies the Unified Thread standards which have been agreed upon by standardization committees of Canada, the United Kingdom and the United States and signalized by the signing of an Accord at Washington, D.C. on November 18, 1948. These Unified standards constitute the basic American standards. The earlier American standards are, in part, continued herein. In order to distinguish clearly between the Unified standards and those which are American standards, both of which appear together in many of the numerical tables, different type faces are employed.