La Spezia-Rimini Line

The line runs through northern Italy, from the cities of La Spezia to Rimini. North and west of the line, the plural of nouns was drawn from the Latin accusative case, and usually ends in -s regardless of grammatical gender or declension. South and east of the line, the plurals of nouns were usually taken from the Latin nominative case, and change the vowels to form the plurals. Compare the plurals of cognate nouns in Italian and Spanish:

Another isogloss that falls on the La Spezia-Rimini line deals with the voicing of certain consonants that occur between vowels. Thus, Latin focus/focum (meaning "fire") becomes fuoco in Italian, but fuego in Spanish. Voicing, softening, or loss of these consonants is characteristic of the western branch of Romance; their retention is characteristic of eastern Romance. Instability of short vowels is also a trait of western Romance; as shown above, Latin lupus/lupum becomes lobo in Spanish, showing both voicing and the merger of short Latin /u/ and /o/, while in Italian the short /u/ is preserved. Generally speaking the western Romance languages show common innovations that the eastern Romance languages tend to lack.