the margin, while in recent years, the lower left margin has become a common place to include the year of issue. Philatelists count changes in these marginal inscriptions as distinct types of stamps. Hidden elements and "secret marks" Sometimes designers include tiny elements into a design, sometimes at the request of the stamp-issuing authority, sometimes on their own. Stamps may have a year or name worked into a design, while the US stamp honoring Rabbi Bernard Revel has a minute Star of David visible in his beard. Secret marks are small design alterations added to distinguish printings unambiguously. These usually take the form of small lines or marks added to clear areas of a design. Chinese stamps of the 1940s have secret marks in the form of slightly altered characters, where two arms might be changed to touch, when previously they were separate. Shape and size The usual shape of a postage stamp is a rectangle, this being an efficient way to pack stamps on a sheet. A rectangle wider than tall is called a "horizontal design", while taller than wide is a "vertical design". A number of additional shapes have been used, including triangles, rhombuses, octagons, circles, and various freeform shapes including heart shapes, and even a banana shaped stamp issued by Tonga from 1969 to 1985. The usual size ranges from 10 to 30 mm in each direction, experience having shown this to be the easiest to handle. Many countries use only a limited selection of dimensions, to simplify automated machinery that handles stamps. The smallest postage stamp on record was issued by Mecklenburg-Schwerin in 1856, and was a square, with sides measuring 10 millimeters. The biggest stamps in history were used in the United States from 1865 and measured 52 by 95 millimeters, but were used exclusively for mailing newspapers. Design evolution Stamp design has undergone a gradual process of evolution, traceable both to advances in printing technology and general changes in taste. Design "fads" may also be observed, where a number of countries tend to imitate each other. This may be driven by printing houses, many of which design and print stamps for multiple countries. For instance, although multi-color printing was always possible, and may be seen on the earliest stamps of Switzerland, the process was slow and expensive, and most stamps were in one or two colors until the 1960s. From time to time postal