This specification states requirements and gives recommendations for suitable steel structures for drilling and well servicing operations in the petroleum industry, provides a uniform method of rating the structures, and provides two product specification levels (PSLs). This specification is applicable to all new steel derricks, masts (including masts with guy lines and service rig masts), substructures, and crown block assemblies with a date of manufacture after the effective date of this specification. Annex A provides standard supplementary requirements (SRs) that apply only if specified by the purchaser. Annex B is an informative annex to assist in an understanding/ application of this API specification. Annex C is an informative annex regarding the API Monogram Program and the API Monogram marking requirements. Annex D is an informative annex providing guidelines to assist the Purchaser with purchasing equipment manufactured to the requirements in this API document.