Linguistic typology & typological universals
Let us begin by considering the issue of linguistic universals. What exactly do linguists mean when they speak of linguistic universals? Well, depending on the approach we take, the term may include a variety of meanings. Taking a typological approach, a linguistic universal is defined very generally as: a statement that is true for all natural languages.
Linguistic typology is a branch of linguistics that studies and classifies languages according to their structural features. Its aim is to describe and explain the structural diversity and the similarities of the world's languages. However, typologists do not try to analyse and compare ALL the different languages of the world, instead, they typically compare around 100-200 languages to see whether they can find certain patterns.
Typologists usually distinguish between unrestricted and implicational universals. Unrestricted (or absolute) universals are patterns that - with respect to some structural features - are true for all languages, i.e. apply to all known languages. Examples of unrestricted universals would be: all spoken languages have nouns and verbs or all languages have oral vowels.
Implicational universals do not state that all languages show the same pattern with respect to a given phenomenon, but instead refer to restrictions on the logically possible patterns, usually in the format: “If language X has property Y, then it will also have property Z”. Implicational universals, in other words, refer to a particular feature that is always accompanied by another feature.