In historical linguistics, cognates or lexical cognates are sets of words in different languages that have been inherited in direct descent from an etymological ancestor in a common parent language. Because language change can have radical effects on both the sound and the meaning of a word, cognates may not be obvious, and often it takes rigorous study of historical sources and the application of the comparative method to establish whether lexemes are cognate or not. Cognates are distinguished from loanwords, where a word has been borrowed from another language.
The term cognate derives from the Latin noun cognatus 'blood relative'.
Cognates need not have the same meaning, which may have changed as the languages developed independently. For example English starve and Dutch sterven 'to die' or German sterben 'to die' all descend from the same Proto-Germanic verb, *sterbaną 'to die'.
Cognates also do not need to look or sound similar: English father, French père, and Armenian հայր (hayr) all descend directly from Proto-Indo-European *ph₂tḗr. An extreme case is Armenian երկու (erku) and English two, which descend from Proto-Indo-European *dwóh₁; the sound change *dw > erk in Armenian is regular.
An example of cognates from the same Indo-European root are: night (English), nicht (Scots), Nacht (German), nacht (Dutch, Frisian), nag (Afrikaans), Naach (Colognian), natt (Swedish, Norwegian), nat (Danish), nátt (Faroese), nótt (Icelandic), noc (Czech, Slovak, Polish), ночь, noch (Russian), ноќ, noć (Macedonian), нощ, nosht (Bulgarian), ніч, nich (Ukrainian), ноч, noch/noč (Belarusian), noč (Slovene), noć (Serbo-Croatian), nakts (Latvian), naktis (Lithuanian), νύξ, nyx (Ancient Greek), νύχτα / nychta (Modern Greek), nakt- (Sanskrit), natë (Albanian), nox, gen. sg. noctis (Latin), nuit (French), noche (Spanish), nueche (Asturian), noite (Portuguese and Galician), notte (Italian), nit (Catalan), nuet/nit/nueit (Aragonese), nuèch / nuèit (Occitan) and noapte (Romanian). These all meaning 'night' and derive from the Proto-Indo-European *nókʷts 'night'. The Indo-European languages have many hundreds of such cognate sets, though few of them are as neat as this.
The Arabic سلام salām, the Hebrew שלום shalom, the Assyrian Neo-Aramaic shlama and the Amharic selam 'peace' are also cognates, derived from the Proto-Semitic *šalām- 'peace'.
Main article: False cognate
False cognates are pairs of words that appear to have a common origin, but which in fact do not. For example, Latin habēre and German haben both mean 'to have' and are phonetically similar. However, the words evolved from different Proto-Indo-European (PIE) roots: haben, like English have, comes from PIE *kh₂pyé- 'to grasp', and has the Latin cognate capere 'to seize, grasp, capture'. Habēre, on the other hand, is from PIE *gʰabʰ 'to give, to receive', and hence cognate with English give and German geben.
Likewise, English much and Spanish mucho look similar and have a similar meaning, but are not cognates: much is from Proto-Germanic *mikilaz < PIE *meǵ- and mucho is from Latin multum < PIE *mel-. A true cognate of much is the archaic Spanish maño 'big'.
Cognates are distinguished from other kinds of relationships.
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