of the Apennine Mountains, which cuts straight across northern Italy and forms a major geographic barrier to further language spread. This would explain why some of the "northwest" features (almost all of which can be characterized as innovations) end at differing points in northern Italy, and why some of the languages in geographically remote parts of Spain (in the south, and high in the Pyrenees) are lacking some of these features. It also explains why the languages in France (especially standard French) seem to have innovated earlier and more extensively than other Western Romance languages. Many of the "southeast" features also apply to the Eastern Romance languages (particularly, Romanian), despite the geographic discontinuity. Examples are lack of lenition, maintenance of intertonic vowels, use of vowel-changing plurals, and palatalization of /k/ to . This has led some researchers, following Walther von Wartburg, to postulate a basic two-way east–west division, with the "Eastern" languages including Romanian and central and southern Italian, although this view is troubled by the contrast of numerous Romanian phonological developments with those found in Italy below the La Spezia-Rimini line. Among these features, in Romanian geminates reduced historically to single units, and /kt/ developed into /pt/, whereas in central and southern Italy geminates are preserved and /kt/ underwent assimilation to /tt/. Despite being the first Romance language to diverge from spoken Latin, Sardinian does not fit at all into this sort of division. It is clear that Sardinian became linguistically independent from the remainder of the Romance languages at an extremely early date, possibly already by the first century BC. Sardinian contains a large number of archaic features, including total lack of palatalization of /k/ and /ɡ/ and a large amount of vocabulary preserved nowhere else, including some items already archaic by the time of Classical Latin (first century BC). Sardinian has plurals in /s/ but post-vocalic lenition of voiceless consonants is normally limited to the status of an allophonic rule (e.g. [k]ane 'dog' but su [ɡ]ane or su [ɣ]ane 'the dog'), and there are a few innovations unseen elsewhere, such as a change of /au/ to /a/. Use of su < ipsum as an article is a retained archaic feature that also exists in the Catalan of the Balearic Islands and that used to be more widespread in Occitano-Romance, and is known as (literally the "salted article"), while Sardinian shares develarisation of earlier /kw/ and /ɡw/ with