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The Northwestern Proto-Indo-European language is the hypothesized proto-language, a descendant of late Proto-Indo-European, which is believed to have given rise to the Celtic, Italic, and Germanic branches of Indo-European languages. There is no complete consensus on its existence or whether a supposed Northwestern Indo-European group constitutes a clade that is phylogenetically justified. However, various scholars argue that the closer lexical proximity of Germanic, Celtic, and Italic, along with certain common features, supports its validity.^[1] Additionally, there are extra-linguistic archaeological and genetic evidences that are compatible with the existence of Northwestern Proto-Indo-European.

Regarding linguistic evidence, studies on hydronymy, as reflected in the so-called Old European hydronymy^[2] (Krahe 1964, 1949; Nicolaisen 1957), reveal an almost uniform system of river naming that displays Indo-European water-related words and suffixes resembling late Proto-Indo-European.^[3][4]

The diffusion area or urheimat of Northwestern Proto-Indo-European is thought to be in Central Europe. It has been proposed that populations associated with the eastern part of the Bell Beaker culture, particularly in the period 2400–1800 BC, may have been linked to a possible Northwestern Proto-Indo-European. This culture later evolved into the Únětice culture (2300–1600 BC), whose chronology aligns well with a stage preceding the split between Proto-Italo-Celtic and Proto-Germanic. In the northern region, the Únětice culture is believed to have given rise to the Nordic Bronze Age (1750–500 BC) and the Jastorf culture (600–1 BC), the latter commonly associated with clearly Germanic-speaking peoples. Slightly further west, the Únětice culture eventually developed into the Tumulus culture (1600–1200 BC), the Urnfield culture (1250–750 BC), and the Terramare culture (1500–1100 BC), which may have been associated with Proto-Italo-Celtic.

Later, the Hallstatt C and D cultures (750–450 BC) are usually identified with Proto-Celtic peoples, as is the La Tène culture, which represents a later development. The Italic peoples may be associated with several archaeological cultures linked to the Urnfield tradition, which spread from northern Italy. Proto-Italics have been linked to the final phase of the Urnfield culture in Italy, known as the Proto-Villanovan culture (1200–900 BC). Although the Villanovan culture itself is considered a direct predecessor of the Etruscan culture, it may have also encompassed speakers of Italic languages.

Archaeogenetic evidence suggests that around 2500 BC, male individuals carrying the Y-chromosome haplogroup Haplogroup R1b (Y-DNA) began to appear across Western Europe. These individuals appear linked to the archaeogenetic lineage of the Western Steppe Herders from the Pontic steppe. Before the 16th century BC, individuals with steppe ancestry were rare, but during the Chalcolithic, they became more common in Western Europe, often associated with bronze weaponry and possibly arriving in horse-drawn carts. The replacement of local Early European Farmers' genetic lineage was so significant that today, the R1b haplogroup, introduced by these Pontic steppe migrants, reaches frequencies of up to 90% in regions closest to the Atlantic Ocean. The genetic imprint of these individuals remains highly significant today. Given that Indo-European languages replaced most pre-Indo-European Neolithic languages, it seems reasonable to associate this genetic replacement with migrations from the steppe, particularly those linked to the Yamnaya culture, which is considered the central diffusion point of Indo-European languages. The R1b haplogroup is mainly associated with the most westerly Indo-European languages, whereas in Eastern Europe, Anatolia, Central Asia, the Near East, South Asia, and parts of Africa, such as Algerian Arabs, where it reaches 10%, different subclades of R1b predominate.^[5]

Among ancient speakers of Italic, Celtic, and Germanic languages, R1b was also very common. However, in present-day Germany, there is also a significant presence of Haplogroup R1a, which was likely more widespread in the Corded Ware culture (and remains frequent among Balto-Slavic populations today). Specifically, Italic, Celtic, and Germanic peoples exhibit a higher proportion of R1b than other Eastern Indo-European populations. This, along with the greater number of shared cognates among Western Indo-European languages, supports the hypothesis of a Northwestern Proto-Indo-European language distinct from the immediate ancestor of the Helleno-Armenian, Indo-Iranian, or Balto-Slavic branches. Additionally, the presence of similar toponyms reflected in Old European hydronyms suggests a certain linguistic homogeneity in Western Europe at the beginning of the 2nd millennium BC. These hydronyms may either stem directly from Northwestern Proto-Indo-European or from an ancient European language directly descended from it.

Thus, archaeological and genetic data provide a basis for the hypothesis that many Bronze Age cultures had a steppe origin and that their languages belonged to the Indo-European family.

In Swadesh list, the percentage of cognates between Proto-Italic and Proto-Celtic exceeds 40%, the same as between Proto-Italic and Proto-Germanic (the percentage of cognates between Proto-Celtic and Proto-Germanic in that list is slightly below 40%). Furthermore, considering broader vocabulary lists, it has been confirmed that Germanic languages share more lexical items with the Italic family than with Celtic languages.^[6] The morphological features between Italic and Celtic have been widely discussed, leading to the general assumption of a closer connection between these two groups. A 2002 study by Ringe, Warnow, & Taylor provided evidence in favor of the "Italo-Celtic" hypothesis.^[7] The Dutch scholar Frederik H. H. Kortlandt has been working on the Italo-Celtic relationship and, in 2007, attempted the linguistic reconstruction of a Proto-Italo-Celtic language.^[8] Other languages, not properly Celtic or Italic, such as Lusitanian and perhaps ancient Ligurian, could be assigned to this group.

The consonantal inventory of Northwestern Proto-Indo-European is similar to that of Proto-Celtic, Proto-Italic, and Proto-Germanic and is reconstructed as follows:

	Bilabial	Dental	Alveolar	Palatal	Velar	Labiovelar
Plosive	*p  b	*t  *d			*k  *ɡ	*kʷ  *ɡʷ
Fricative	*β	*ð	*s		*ɣ	*ɣʷ
Nasal	*m	*n			(*ŋ)
Sonorant		*r	*l	*j		*w

The phonetic evolution would have been as follows:

• The voiced aspirates of Indo-European /*bʰ, *dʰ, *gʰ, *gʷʰ/ are hypothesized to have developed into voiced fricatives /*β, *ð, *ɣ, *ɣʷ/. The first three remained as such in Germanic (although transcribed as /*b, *d, *g/, evidence suggests these sounds were voiced fricatives in Germanic). In Italic, /*β, *ð, *ɣ/ remained unchanged in medial position but devoiced in other contexts, yielding the allophones [*ɸ, *θ, *x]. As for /*ɣʷ/, Latin developed it into medial /gʷ/ gu- or /w-/ in initial position. In Proto-Celtic, /*ɣʷ/ also became /*gʷ/.
• The plosives /*p, *b, *t, *d, *k, *g, *kʷ, *gʷ/ remained unchanged. Later, Celtic underwent the shift /*p/ > /*ɸ/ > ${\displaystyle \varnothing }$ and /*gʷ/ > /*b/. In Germanic, the First Consonant Shift led to the devoicing of voiced plosives and the fricativization of voiceless plosives. In Northwestern Proto-Indo-European, pIE consonants reconstructed as /*ḱ, *ǵ/ merged with the outcome of pIE /*k, *g/.

The nominal declension varies slightly depending on the thematic vowel and whether it belongs to the athematic inflection. This variation is still observable in Germanic, Italic, and Celtic, suggesting it originates from their common ancestor. The thematic inflection system included the following cases:

The verbal inflection distinguishes between an active voice (with endings common to all three branches) and a mediopassive voice (where Italic and Celtic share similar endings). Additionally, Germanic reconstructs dual forms, whereas Italic preserves only singular and plural. The primary, secondary, and stative endings for all three branches are as follows:

Primary endings are used with the present, secondary endings with the imperfect, and stative endings are typically used in the perfect tense.

The numerals reconstructed for the three branches descending from Northwestern Proto-Indo-European (pNWIE), along with the original reconstructed form, are:

Other reconstructible terms from the shared vocabulary, which appear in the Swadesh list, are the following terms:

When any of the terms in a row is not cognate, it is indicated in parentheses.

Some additional terms that do not appear in the short Swadesh list are:

• Adrados, F. R. (1998). Adrados, F. R.; Bernabé, A.; Mendoza, J. (eds.). La reconstrucción del indoeuropeo y de su diferenciación dialectal. Madrid, Spain: Ediciones Clásicas.
• Mallory, J. P. (2013). "The Indo-Europeanization of Atlantic Europe". In Koch, John T.; Cunliffe, Barry (eds.). Celtic from the West 2: Rethinking the Bronze Age and the Arrival of Indo-European in Atlantic Europe. Oxford, UK: Oxbow Books. pp. 17–40. ISBN 978-1-84217-529-3.