Roman Numerals
Converts Arabic numbers to Roman numerals and back, from 1 to 3,999,999: above 3,999 an overline (vinculum) multiplies a numeral by 1,000. Non-standard input such as IIII is read and its standard form shown, with a breakdown of the parts. Florian Cajori (1928) notes that the subtractive rule (IV for IIII) was seldom applied by the Romans, and that a line over a numeral for thousands appears in late antiquity.