In linguistics, more precisely in traditional grammar, a distributive number is a word that answers "how many times each?" or "how many at a time?", such as singly or doubly. They are contrasted with multipliers. In English, this part of speech is rarely used and much less recognized than cardinal numbers and ordinal numbers, but it is clearly distinguished and commonly used in Latin and several Romance languages, such as Romanian.
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English
In English distinct distributive numbers exist, such as singly, doubly, and triply, and are derived from the corresponding multiplier (of Latin origin, via French) by suffixing -y (reduction of Middle English -lely > -ly). However, this is more commonly expressed periphrastically, such as "one by one", "two by two"; "one at a time", "two at a time"; "in twos", "in threes"; or using a counter word such as "in groups of two" or "two pieces to a …". Examples include "Please get off the bus one by one so no-one falls.", "She jumped up the steps two at a time.", "Students worked in the lab in twos and threes.", "Students worked in groups of two and three.", and "Students worked two people to a team."
The suffixes -some (as in twosome, threesome) and -fold (as in two-fold, three-fold) are also used, though also relatively infrequently. For musical groups solo, duo, trio, quartet, etc. are commonly used, and pair is used for a group of two.
A conspicuous use of distributive numbers is in arity or adicity, to indicate how many parameters a function takes. Most commonly this uses Latin distributive numbers and -ary, as in unary, binary, ternary, but sometimes Greek numbers are used instead, with -adic, as in monadic, dyadic, triadic.
Other languages
Georgian, Latin, and Romanian are notable languages with distributive numbers; see Romanian distributive numbers.
In Japanese numerals, distributive numbers are formed regularly from a cardinal number, a counter word, and the suffix -zutsu (ずつ), as in hitori-zutsu (一人ずつ, one person at a time, one person each).
In Turkish, one of the -ar/-er suffixes (chosen according to vowel harmony) are added to the end of a cardinal number, as in "birer" (one of each) and "dokuzar" (nine of each). If the number ends with a vowel, a letter ş comes to the middle; as in "ikişer" (two of each) and "altışar" (six of each).