Numeral or number prefixes are prefixes derived from numerals or occasionally other numbers. In English and many other languages, they are used to coin numerous series of words. For example: unicycle, bicycle, tricycle (1-cycle, 2-cycle, 3-cycle) dyad, triad (2 parts, 3 parts) biped, quadruped (2 legs, 4 legs) September, October, November, December (month 7, month 8, month 9, month 10) decimal, hexadecimal (base-10, base-16) septuagenarian, octogenarian (70-79 years old, 80-89 years old) centipede, millipede (around 100 legs, around 1000 legs) In many European languages there are two principal systems, taken from Latin and Greek, each with several subsystems; in addition, Sanskrit occupies a marginal position. There is also an international set of metric prefixes, which are used in the metric system and which for the most part are either distorted from the forms below or not based on actual number words. Table of number prefixes in English In the following prefixes, a final vowel is normally dropped before a root that begins with a vowel, with the exceptions of bi-, which is bis- before a vowel, and of the other monosyllables, du-, di-, dvi-, tri-, which are invariable. The cardinal series are derived from cardinal numbers, such as the English one, two, three. The multiple series are based on adverbial numbers like the English once, twice, thrice. The distributive series originally meant one each, two each or one by one, two by two, etc., though that meaning is now frequently lost. The ordinal series are based on ordinal numbers such as the English first, second, third (for numbers higher than 2, the ordinal forms are also used for fractions; only the fraction has special forms). For the hundreds, there are competing forms: those in -gent-, from the original Latin, and those in -cent-, derived from centi-, etc. plus the prefixes for 1–9. The same suffix may be used with more than one series: {| |-align=center |primary||secondary||tertiary||quartary||quintary||sextary||—||—||nonary||— |-align=center |singulary||binary||ternary, trinary||quaternary||quinary||senary||septenary||octonary||novenary||denary |} Examples Occurrences Numerical prefixes occur in 19th-, 20th-, and 21st-century coinages, mainly the terms that are used in relation to or that are the names of technological innovations, such as hexadecimal and bicycle. Also used in medals that commemorate an anniversary, such as sesquicentennial (150 years), centennial (100 years), or bicentennial (200 years). They occur in constructed words such as systematic names. Systematic names use numerical prefixes derived from Greek, with one principal exception, nona-. They occur as prefixes to