as a 5 × 5 grid consisting of five 5-letter words, thus totaling 25 characters. It uses 8 different Latin letters: 5 consonants (S, T, R, P, N) and 3 vowels (A, E, O). In some versions, the vertical and horizontal lines of the grid are also drawn, but in many cases, there are no such lines. The square is described as a two-dimensional palindrome, or word square, which is a particular class of a double acrostic. The square comes in two forms: ROTAS (left, below), and the SATOR (right, below): The earliest Roman-era versions of the square have the word ROTAS as the top line (called a ROTAS-form square, left above), but the inverted version with SATOR in the top line became more dominant from early medieval times (called a SATOR-form square, right above). Some academics call it a Rotas-Sator Square, and some of them refer to the object as a rebus, or a magic square. Since medieval times, it has also been known as a Templar Magic Square. Discovery and dating The existence of the square was long recognized from early medieval times, and it has been found on the continents of Europe (and Byzantium in Asia Minor), North Africa (mainly Coptic settlements), and the Americas. Medieval examples of the square in SATOR-form abound, including the earliest French example in a Carolingian Bible from A.D 822 at the monastery of Saint-Germain-des-Prés. Many medieval European churches and castles have Sator square inscriptions. The first recognized serious academic study of the square was the 1881 publication of 's historical survey in , titled "Sator-Arepo-Formel", and a considerable body of academic research has been subsequently published on the meaning of the square. Up until the 1930s, a Coptic papyrus with the square in the ROTAS-form dating from the fourth or fifth century A.D was considered the earliest version. In 1889, British ancient historian Francis Haverfield identified the 1868 discovery of a Sator square found in ROTAS-form scratched on a plaster wall in the Roman settlement of Corinium at Cirencester to be of Roman origin, however, his assertion was discounted by academics who considered it an "early medieval charm". Haverfield would be proved right by the 1931-32 excavations at Dura-Europos in Syria that uncovered three Sator square inscriptions, all in ROTAS-form, on the interior walls of a Roman military office (and a fourth a year later) that dated from circa A.D 200.