Kalends, and for the 28 days in February. The latter can be seen on the sole extant pre-Julian calendar, the Fasti Antiates Maiores. Rare variants While irregular subtractive and additive notation has been used at least occasionally throughout history, some Roman numerals have been observed in documents and inscriptions that do not fit either system. Some of these variants do not seem to have been used outside specific contexts, and may have been regarded as errors even by contemporaries. was how people associated with the XXII Roman Legion used to write their number. The practice may have been due to a common way to say "twenty-second" in Latin, namely duo et vice(n)sima (literally "two and twentieth") rather than the "regular" vice(n)sima secunda (twenty second). Apparently, at least one ancient stonecutter mistakenly thought that the of "22nd Legion" stood for 18, and "corrected" it to . There are some examples of year numbers after 1000 written as two Roman numerals 1–99, e.g. 1613 as , corresponding to the common reading "sixteen thirteen" of such year numbers in English, or 1519 as as in French quinze-cent-dix-neuf (fifteen-hundred and nineteen), and similar readings in other languages. In some French texts from the 15th century and later, one finds constructions like for 99, reflecting the French reading of that number as quatre-vingt-dix-neuf (four-score and nineteen). Similarly, in some English documents one finds, for example, 77 written as "" (which could be read "three-score and seventeen"). A medieval accounting text from 1301 renders numbers like 13,573 as "", that is, "13×1000 + 5×100 + 3×20 + 13". Other numerals that do not fit the usual patterns – such as for 45, instead of the usual — may be due to scribal errors, or the writer's lack of familiarity with the system, rather than being genuine variant usage. Non-numeric combinations As Roman numerals are composed of ordinary alphabetic characters, there may sometimes be confusion with other uses of the same letters. For example, "XXX" and "XL" have other connotations in addition to their values as Roman numerals, while "IXL" more often than not is a gramogram of "I excel", and is in any case not an unambiguous Roman numeral. Zero As a non-positional numeral system, Roman numerals have no "place-keeping" zeros. Furthermore, the system as used by the Romans lacked a numeral for the number zero itself (that is, what remains after 1 is subtracted from