was written as . This is a system of encasing numbers to denote thousands (imagine the s and s as parentheses), which has its origins in Etruscan numeral usage. Each additional set of and surrounding raises the value by a factor of ten: represents 10,000 and represents 100,000. Similarly, each additional to the right of raises the value by a factor of ten: represents 5,000 and represents 50,000. Numerals larger than do not occur. Sometimes was reduced to for 1,000. Similarly, for 5,000 was reduced to ; for 10,000 to ; for 50,000 to (ↇ); and (ↈ) for 100,000 to . and most likely preceded, and subsequently influenced, the adoption of "" and "" in Roman numerals. John Wallis is often credited for introducing the symbol for infinity , and one conjecture is that he based it on , since 1,000 was hyperbolically used to represent very large numbers. Vinculum Another system was the vinculum, in which conventional Roman numerals were multiplied by 1,000 by adding a "bar" or "overline", thus: = 4,000 = 25,000 It was a common alternative to the apostrophic ↀ during the Imperial era around the Roman world (M for '1000' was not in use until the Medieval period). It continued in use in the Middle Ages, though it became known more commonly as titulus, and it appears in modern editions of classical and medieval Latin texts. A three-sided box (now sometimes printed as two vertical lines and a vinculum) was used to multiply by 100,000, thus: p. = 1,332,000 paces (1,332 Roman miles). Vinculum notation is distinct from the custom of adding an overline to a numeral simply to indicate that it is a number, and both usages can be seen on Roman inscriptions of the same period and general location, such as on the Antonine Wall. Origin The system is closely associated with the ancient city-state of Rome and the Empire that it created. However, due to the scarcity of surviving examples, the origins of the system are obscure and there are several competing theories, all largely conjectural. Etruscan numerals Rome was founded sometime between 850 and 750 BC. At the time, the region was inhabited by diverse populations of which the Etruscans were the most advanced. The ancient Romans themselves admitted that the basis of much of their civilization was Etruscan. Rome itself was located next to the southern edge of the Etruscan