A sister language is a cognate language, that is, coming from the same once-existing language or hypothetical root language. The latter language is the so-called proto-language

Proto-language

A proto-language in the tree model of historical linguistics is the common ancestor of the languages that form a language family. Occasionally, the German term Ursprache is used instead.Often the proto-language is not known directly...

. There are many examples of sister languages. Many of these examples involve the Romance languages

Romance languages

The Romance languages are a branch of the Indo-European language family, more precisely of the Italic languages subfamily, comprising all the languages that descend from Vulgar Latin, the language of ancient Rome...

, each one of which derives from Latin

Latin

Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

; though they are all somewhat different, they are not linguistically indescernible. Italian

Italian language

Italian is a Romance language spoken mainly in Europe: Italy, Switzerland, San Marino, Vatican City, by minorities in Malta, Monaco, Croatia, Slovenia, France, Libya, Eritrea, and Somalia, and by immigrant communities in the Americas and Australia...

 and French

French language

French is a Romance language spoken as a first language in France, the Romandy region in Switzerland, Wallonia and Brussels in Belgium, Monaco, the regions of Quebec and Acadia in Canada, and by various communities elsewhere. Second-language speakers of French are distributed throughout many parts...

 have about 89% lexical overlap, meaning the words share 89 percent of the same characteristics and root origins. Both Italian and French have a huge number of similar words. Similarly, Spanish

Spanish language

Spanish , also known as Castilian , is a Romance language in the Ibero-Romance group that evolved from several languages and dialects in central-northern Iberia around the 9th century and gradually spread with the expansion of the Kingdom of Castile into central and southern Iberia during the...

 and Portuguese

Portuguese language

Portuguese is a Romance language that arose in the medieval Kingdom of Galicia, nowadays Galicia and Northern Portugal. The southern part of the Kingdom of Galicia became independent as the County of Portugal in 1095...

 have about 89% lexical overlap, so many words are shared or similar between those two languages (see also cognates). Spanish and Romanian's

Romanian language

Romanian Romanian Romanian (or Daco-Romanian; obsolete spellings Rumanian, Roumanian; self-designation: română, limba română ("the Romanian language") or românește (lit. "in Romanian") is a Romance language spoken by around 24 to 28 million people, primarily in Romania and Moldova...

 overlap is lower, at about 67%. Spanish and Portuguese have undergone Arabic

Arabic language

Arabic is a name applied to the descendants of the Classical Arabic language of the 6th century AD, used most prominently in the Quran, the Islamic Holy Book...

 influence and Romanian has undergone many different influences over the years, particularly from the Slavic languages

Slavic languages

The Slavic languages , a group of closely related languages of the Slavic peoples and a subgroup of Indo-European languages, have speakers in most of Eastern Europe, in much of the Balkans, in parts of Central Europe, and in the northern part of Asia.-Branches:Scholars traditionally divide Slavic...

. The Scots language

Scots language

Scots is the Germanic language variety spoken in Lowland Scotland and parts of Ulster . It is sometimes called Lowland Scots to distinguish it from Scottish Gaelic, the Celtic language variety spoken in most of the western Highlands and in the Hebrides.Since there are no universally accepted...

is considered to be a sister language of English, due to the two languages sharing their entire vocabulary, with nothing but spelling and slight aural differenced between the words