often share a common cover-page layout. Technical reports used to be made available in print, but are now more commonly published electronically (typically in PDF), whether on the Internet or on the originating organization's intranet. Several schemes have been proposed or are in use to uniquely identify either an entire report series or an individual report: The entire series may be uniquely identified by an ISSN. Where reports are published both on paper and electronically, two different ISSNs are often assigned to distinguish between these. A Standard Technical Report Number (STRN) identifier scheme, proposed for use by U.S. government agencies, was first defined in 1974, and became U.S. national standard ANSI/NISO Z39.23 in 1983. Such numbers consisted initially of two parts: (1) a report code of alphanumeric characters that designate the issuing organization and series, and (2) a sequential group of numeric characters assigned by the issuing organization. The national maintenance agency for assigning report codes was the National Technical Information Service (NTIS), which also operates the National Technical Reports Library. The Z39.23 standard was further revised in 1990 to allow longer codes and greater variability of separators. This extended format was in 1994 also adopted in ISO 10444 (see below), and remains (after an "ISRN" prefix) in the current version ANSI/NISO Z39.23-1997. An international registration scheme for a globally unique (ISRN) was standardized in 1994 (ISO 10444). It had aimed to be an international extension of the ANSI/NISO Z39.23 scheme. However the registration agency needed for its operation was never implemented in practice. ISO finally withdrew this standard in December 2007. Like many other scientific publications, technical reports are now also commonly uniquely identified via the Digital Object Identifier (DOI) system, which facilitates access via HTTP. DOIs have now in practice largely replaced Z39.23-style standard technical report numbers. See also Grey literature References External links National Technical Information Service National Information Standards Organization American National Standards Institute Department of Defense technical reports at Defense Technical Information Center NASA technical reports Archive of the National Advisory Committee for Aeronautics (NACA) Chartered in 1915, operational from 1917-1958. The National Aeronautics and Space Act of 1958 created NASA from NACA. Technical Report Archive and Image Library (TRAIL) U.S. government technical reports issued primarily prior to 1975 and digitized by the TRAIL. More on TRAIL Scientific documents Technical communication Grey literature