在语言研究中，由连词（通常是and）或介词连接的一对单词（例如，响亮而清晰）被称为二项式或二项式对。

当这对词的词序固定时，二项式被称为不可逆的。

包含三个名词或形容词（bell、book和candle；calme、cool和collected）的类似结构称为三项式。

二项式的常见例子

英语中有许多双音词的例子。下面的例子被认为是不可逆的二项式，因为每对的顺序是固定的。

• 疼痛
• 更大更好
• 面包和黄油
• 停止
• 制衡
• 死的还是活的
• 注意事项
• 公平合理
• 商品和服务
• 火腿蛋
• 高低
• 亲亲抱抱
• 刀叉
• 生死
• 螺母和螺栓
• 苍老
• 针锋相对
• 锅碗瓢盆
• 白手起家
• 兴衰
• 起立

可逆和不可逆二项式

虽然有些二项式是不可逆的，但有些二项式是可以颠倒的。这两个词之间的主要区别在于，当两个词颠倒时，可逆的双音词听起来并不奇怪；而不可逆二项式在对的顺序切换时听起来很尴尬。

"In the typical newspaper headline Cold and snow grip the nation it is proper to set off the segment cold and snow as a binomial, if one agrees so to label the sequence of two words pertaining to the same form-class, placed on an identical level of syntactic hierarchy, and ordinarily connected by some kind of lexical link. There is nothing unchangeable or formulaic about this particular binomial: Speakers are at liberty to invert the succession of its members (snow and cold . . .) and may with impunity replace either snow or cold by some semantically related word (say, wind or ice). However, in a binomial such as odds and ends the situation is different: The succession of its constituents has hardened to such an extent that an inversion of the two kernels--*ends and odds--would be barely understandable to listeners caught by surprise. Odds and ends, then, represents the special case of an irreversible binomial." (Yakov Malkiel, "Studies in Irreversible Binomials." Essays on Linguistic Themes. University of California Press, 1968)

同义双音词和回声双音词

同义双义词是两个具有相同或相似含义的词对。回声双音词是两个相同的词。

"The third most frequent binomial in the DoD [Department of Defense] corpus is 'friends and allies,' with 67 instances. Unlike the majority of binomials, it is reversible: 'allies and friends' also occurs, with 47 occurrences. "Both allies and friends refer to countries which accord with US policies; as such, the two coordinates of the binomial may incline us to categorize the binomial as 'synonymous' (Gustafsson, 1975). Rhetorically speaking, friends and allies may have an intensifying function, similar to 'echoic' binomials (where WORD1 is identical to WORD2), such as more and more and stronger and stronger." (Andrea Mayr, "Language and Power: An Introduction to Institutional Discourse." Continuum, 2008)