Editing Comparative method (section)

===Terminology===
''Descent'' is defined as transmission across the generations: children learn a language from the parents' generation and, after being influenced by their peers, transmit it to the next generation, and so on. For example, a continuous chain of speakers across the centuries links [[Vulgar Latin]] to all of its modern descendants.

Two languages are ''[[genetic (linguistics)|genetically]] related'' if they descended from the same [[proto-language|ancestor language]].<ref>{{harvnb|Lyovin|1997|pp=1–2}}.</ref> For example, [[Italian language|Italian]] and [[French language|French]] both come from [[Latin]] and therefore belong to the same family, the [[Romance languages]].<ref>{{harvnb|Beekes|1995|p=25}}.</ref> Having a large component of vocabulary from a certain origin is not sufficient to establish relatedness; for example, heavy [[loanword|borrowing]] from [[Arabic]] into [[Persian language|Persian]] has caused more of the [[vocabulary]] of Modern Persian to be from Arabic than from the direct ancestor of Persian, [[Proto-Indo-Iranian]], but Persian remains a member of the Indo-Iranian family and is not considered "related" to Arabic.<ref>{{harvnb|Campbell|2000|p=1341}}</ref>

However, it is possible for languages to have different degrees of relatedness. [[English language|English]], for example, is related to both [[German language|German]] and [[Russian language|Russian]] but is more closely related to the former than to the latter. Although all three languages share a common ancestor, [[Proto-Indo-European]], English and German also share a more recent common ancestor, [[Proto-Germanic]], but Russian does not. Therefore, English and German are considered to belong to a subgroup of Indo-European that Russian does not belong to, the [[Germanic languages]].<ref>{{harvnb|Beekes|1995|pp=22, 27–29}}.</ref>

The division of related languages into subgroups is accomplished by finding ''shared linguistic innovations'' that differentiate them from the parent language. For instance, English and German both exhibit the effects of a collection of sound changes known as [[Grimm's Law]], which Russian was not affected by. The fact that English and German share this innovation is seen as evidence of English and German's more recent common ancestor—since the innovation actually took place within that common ancestor, before English and German diverged into separate languages. On the other hand, ''shared retentions'' from the parent language are not sufficient evidence of a sub-group. For example, German and Russian both retain from Proto-Indo-European a contrast between the [[dative case]] and the [[accusative case]], which English has lost. However, that similarity between German and Russian is not evidence that German is more closely related to Russian than to English but means only that the ''innovation'' in question, the loss of the accusative/dative distinction, happened more recently in English than the divergence of English from German.