great variety of stamps in different styles and sizes (often large), typically depicting persons, places, objects or events connected with the country and its history. The stamps were mostly printed by photogravure, but lithography reappeared and became common for a number of issues beginning in 1992. Mexico's stamps were printed in one or two colors until the stamps commemorating the 1968 Summer Olympics held in Mexico City in 1968, which were the first multicolored stamps issued by Mexico. These stamp issues included several series of stamps with common design elements which were added to over a period of time, especially as inflation increased postal rates. Several of these series have been popular with philatelists, foremost of which is the Exporta issue, discussed below. Annual stamp production increased significantly during the late 20th century. See also Eppens Gallery of Postal Stamps Architecture and Archaeology series From 1950 to 1975, Mexico issued a series of small format definitive stamps with similar basic design for surface mail. Although this series is known by philatelists as the "Architecture and Archaeology" series, it in fact included some other subjects such as the centennial of the Mexican constitution. Many of stamps The "Architecture and Archaeology" series also included a number of larger format airmail stamps (1950–1976) with common design, although some specialists treat the two series separately. The airmail series had even more varieties than the surface mail stamps, some of which are fairly rare, selling for over $100 each. Pulver has stated that this issue "is made to order for a modern specialist, with collecting challenge comparable to a 19th-century classic issue." Some of the later issues in this series were printed on light-active paper, coated with optical brighteners causing it to fluoresce under ultraviolet light, or which had phosphorescent tagging added. These coatings were used for security purposes and to facilitate high speed machine sorting. etiangui ( Enrique Sanchez Garcia ) "The Arqueologia y Arquitectura serie, complete classification guide" Exporta series From 1975 to 1993, Mexico issued a series of definitive regular and airmail stamps in a uniform style depicting a great variety of products Mexico exports, such as beef, bicycles. tomatoes and chemicals, each stamp bearing the Exporta Logo. The series was added to over the years, and there are a great number of variants of papers, sizes, colors, watermarks, and plate flaws. A number of the stamps had burelage printed on their