A technical standard can also be a controlled artifact or similar formal means used for calibration. Reference Standards and certified reference materials have an assigned value by direct comparison with a reference base. A primary standard is usually under jurisdiction of a national standards body. Secondary, tertiary, check standards and standard materials may be used for reference in a metrology system. This article discusses formal technical standards. A custom, convention, company product, corporate standard, etc which becomes generally accepted and dominant is often called a de facto standard. A technical standard can be developed privately or unilaterally, for example by a corporation, regulatory body, military, etc. Standards can also be developed by groups such as trade unions, and trade associations. Standards organizations usually have more diverse input and usually develop voluntary standards: these might become mandatory if adopted by a government, business contract, etc.