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Historically, the La Spezia–Rimini Line marked a series of isoglosses that distinguished Northern Italian speech from that of Tuscany, home of the standard Italian language.
North and west of the line (excluding all Northern Italian varieties) the plural of nouns was drawn from the Latinaccusative case, and is marked with /s/ regardless of grammatical gender or declension. South and east of the line, the plurals of nouns are marked by changing the final vowel, either because these were taken from the Latin nominative case, or because the original /s/ changed into a vocalic sound (see the Romance plurals debate). Compare the plurals of cognate nouns in Aromanian, Romanian, Italian, Spanish, Portuguese, Catalan, French, Sardinian and Latin:
Another isogloss that falls on the La Spezia–Rimini Line deals with the restructured voicing of voiceless consonants, mainly Latin sounds /p/, /t/ and /k/, which occur between vowels. Thus, Latin catēna ('chain') becomes catena in Italian, but cadeia in Portuguese, cadena in Catalan and Spanish, cadéna/cadèina in Emilian, caéna/cadéna in Venetian and chaîne in French. Voicing, or further weakening, even to loss of these consonants is characteristic of the western branch of Romance; their retention is characteristic of eastern Romance.
However, the differentiation is not totally systematic, and there are exceptions that undermine the isogloss: Gascon dialects in south-west France and Aragonese in northern Aragon, Spain (geographically Western Romance) also retain the original Latin voiceless stop between vowels. The presence in Tuscany and elsewhere below the line of a small percentage but large number of voiced forms both in general vocabulary and in traditional toponyms also challenges its absolute integrity.
The criterion of preservation vs. simplification of Latin geminate consonants stands on firmer ground. The simplification illustrated by Castilian boca /boka/ 'mouth' vs. Tuscan bocca /bokka/, both continuations of Latin bucca, typifies all of Western Romance and is systematic for all geminates except /rr/ in some locales (e.g. Spanish carro and caro are still distinct), and to some degree for earlier /ll/ and /nn/ which, while not preserved as geminates, did not generally merge with their simplex correlates (e.g. /n/ > /n/ but /nn/ > /ɲ/ in Spanish, annus > /aɲo/ 'year'). Nevertheless, the La Spezia-Rimini line is real in this respect for most of the consonant inventory, although it should be noted that simplification of geminates to the east in Romania spoils the neat east-west division.
Indeed, the significance of the La Spezia–Rimini Line is often challenged by specialists within both Italian dialectology and Romance dialectology. One reason is that while it demarcates preservation (and expansion) of phonemic geminate consonants (Central and Southern Italy) from their simplification (in Northern Italy, Gaul, and Iberia), the areas affected do not correspond consistently with those defined by voicing criterion. Romanian, which on the basis of lack of voicing is classified with Central and Southern Italian, has undergone simplification of geminates, a defining characteristic of Western Romance.