Linking elements occur in compound nouns and derivatives in the Indo-European languages as well as in many other languages of the world. They can be described as sound material or graphemes with or without a phonetic correspondence appearing between two parts of a word-formation product. Linking elements are meaningless per definition. However, in many cases the clear-cut distinction between them and other, meaningful elements (like inflectional or derivational affixes) is difficult. Here, a thorough examination is necessary.
Simple rules cannot describe the occurrence of linking elements. Instead, their distribution is fully erratic or at least complex, as different factors including the prosodic, morphological, or semantic properties of the word-formation components play a role and compete. The same holds for their productivity: their ability to appear in new word-formation products differs considerably and can range from strongly (prosodically, morphologically, or lexically) restricted to the virtual absence of any constraints.
Linking elements should be distinguished from singular, isolated insertions (cf. Spanish rousseau-n-iano) or extensions of one specific stem or affix (cf. ‑l- in French congo-l-ais, togo-l-ais, English Congo-l-ese, Togo-l-ese). As they link two parts of a word formation, they also differ from word-final elements attached to compounds like ‑(s)I in Turkish as in ana‑dil‑i (mother‑tongue‑i) ‘mother tongue’. Furthermore, they are also distinct from infixes, i.e., derivational affixes that are inserted into a root, as well as from confixes, which are for bound, but meaningful (lexical) morphemes.
Linking elements are attested in many Indo-European languages (Slavic, Romance, Germanic, Baltic languages, and Greek) as well as in other languages across the world. They seem to be more common in compounds than in derivatives. Additionally, some languages display different sets of linking elements in both compounds and derivatives. The linking inventories differ strongly even between closely related languages. For example, Frisian and Dutch, each of which has five different linking elements, share only two linking forms (‑s- and ‑e-).
In some languages, linking elements are homophonous to other (meaningful) elements, e.g., inflectional or derivational suffixes. This is mostly due to their historical development and to the degree of the dissociation from their sources. This makes it sometimes difficult to distinguish between linking elements and meaningful elements. In such cases (e.g., in German or Icelandic), formal and functional differences should be taken into account. It is also possible that the homophony with the inflectional markers is incidental and not a remnant of a historical development. Generally, linking elements can have different historical sources: primary suffixes (e.g., Lithuanian), case markers (e.g., many Germanic languages), derivational suffixes (e.g., Greek), prepositions (e.g., Sardinian and English). However, the historical development of many linking elements in many languages still require further research.
Depending on their distribution, linking elements can have different functions. Accordingly, the functions strongly differ from language to language. They can serve as compound markers (Greek), as “reopeners” of closed stems for further morphological processes (German), as markers of prosodically and/or morphologically complex first parts (many Germanic languages), as plural markers (Dutch and German), and as markers of genre (German).