1. Terminology

Linguists distinguish between two kinds of universals: absolute (opposite: statistical, often called tendencies) and implicational (opposite non-implicational). Absolute universals apply to every known language and are quite few in number; an example would be All languages have pronouns. An implicational universal applies to languages which have a particular feature that is always accompanied by another feature, such as If a language has trial grammatical number, it also must have dual grammatical number, while non-implicational universals just state the existence (or non-existence) of one particular feature.

Also in contrast to absolute universals are tendencies, statements that may not be true for all languages, but nevertheless are far too common to be the result of chance. They also have implicational and non-implicational forms. An example of the latter would be The vast majority of languages have nasal consonants ^[1].  However, most tendencies, like their universal counterparts, are implicational. For example, With overwhelmingly greater than chance frequency, languages with normal SOV order are postpositional. Strictly speaking, a tendency is not a kind of universal, but exceptions to most statements called universals can be found. For example, Latin is an SOV language with prepositions. Often it turns out that these exceptional languages are undergoing a shift from one type of language to another. In the case of Latin, its descendant Romance languages switched to SVO, which is a much more common order among prepositional languages.

Universals may also be bidirectional or unidirectional. In a bidirectional universal two features each imply the existence of each other. For example, languages with postpositions usually have SOV order, and likewise SOV languages usually have postpositions. The implication works both ways, and thus the universal is bidirectional. By contrast, in a unidirectional universal the implication only works one way. Languages which place relative clauses before the noun they modify again usually have SOV order, so prenominal relative clauses imply SOV. On the other hand, worldwide SOV languages show little preference for prenominal relative clauses, and thus SOV implies little about the order of relative clauses. As the implication only works one way, the proposed universal is a unidirectional one.

Linguistic universals in syntax are sometimes held up as evidence for universal grammar (though epistemological arguments are more common). Other explanations for linguistic universals have been proposed, for example that linguistic universals tend to be properties of language which aid communication. If a language were to lack one of these properties, it has been argued, it would probably soon evolve into a language having that property.

2. In Phonology

Some linguistic universals (or tendencies) may be phonologically based. For example, the raspberry sound is never used phonemically within any human language, while there is no known language without /a/ or /ɑ/. Also, all languages have some plosives.

High pitch sounds (such as /i/, as in poquito (Spanish), kitty (English), maomi (Mandarin)) frequently refer to smaller beings or objects (supposed to be such since smaller creatures can only produce higher pitched sounds).

3. In Semantics

In the domain of semantics, research into linguistic universals has taken place in a number of ways. Some linguists, starting with Leibniz, have pursued the search for a hypothetic irreducible semantic core of all languages. A modern variant of this approach can be found in the Natural Semantic Metalanguage of Wierzbicka and associates ^[2]. Other lines of research suggest cross-linguistic tendencies to use body part terms metaphorically as adpositions,^[3] or tendencies to have morphologically simple words for cognitively salient concepts.^[4] The human body, being a physiological universal, provides an ideal domain for research into semantic and lexical universals. In a seminal study, Cecil H. Brown (1976) proposed a number of universals in the semantics of body part terminology, including the following: in any language, there will be distinct terms for BODY, HEAD, ARM, EYES, NOSE, and MOUTH; if there is a distinct term for FOOT, there will be a distinct term for HAND; similarly, if there are terms for INDIVIDUAL TOES, then there are terms for INDIVIDUAL FINGERS. Subsequent research has shown that most of these features have to be considered cross-linguistic tendencies rather than true universals. Several languages, for example Tidore and Kuuk Thaayorre, lack a general term meaning 'body'. On the basis of such data it has been argued that the highest level in the partonomy of body part terms would be the word for 'person'.^[5]