Italo-Western is, in some classifications, the largest branch of the Romance languages. It comprises two of the branches of Romance languages: Italo-Dalmatian and Western Romance. It excludes the Sardinian language and Eastern Romance.
mutual intelligibility, Dalby lists four languages: Corsican, Tuscan and Italian, Napolitan- Sicilian, and Dalmatian.
Tuscan and Italian
Tuscan language is mainly spoken in Tuscany, Italy.
The Italian language, is the closest to Latin of the Romance languages. Italian is an official language in  Italy, Switzerland, San Marino, Vatican City and western Istria (in Slovenia and Croatia). It used to have official status in Albania, Malta and Monaco, where it is still widely spoken, as well as in former Italian East Africa and Italian North Africa regions where it plays a significant role in various sectors. Italian is also spoken by large expatriate communities in the Americas and Australia. Italian has been primarily based on Tuscan, specifically on its Florentine variety and later deeply influenced by almost all regional languages of Italy while its received pronunciation (known as Pronuncia Fiorentina Emendata, Amended Florentine Pronunciation) is based on the accent of Romanesco ( Roman dialect); these are the reasons why Italian differs significantly from Tuscan. 
Neapolitan language is spoken in Southern Italy.
The Campanian dialect is spoken in the southern part of
Lazio and in the region of Campania. The Abruzzese dialect.
The Apulian dialect.
The Northern Calabrian-Lucanian dialect.
The Molisan dialect. The Italian of the Far South:
Judeo-Italian languages are varieties of Italian used by Jewish communities, between the 10th and the 20th centuries, in Italy, Corfu and Zante.
Western Romance languages
mutual intelligibility, Dalby lists a dozen languages: Portuguese, Spanish, Asturian-Leonese, Aragonese, Catalan, Gascon, Provençal, Gallo-Wallon, French, Franco-Provençal, Romansh, and Ladin.
Gallo-Romance can include:
Rhaeto-Romance languages. They include Romansh of Switzerland, Ladin of the Dolomites area, Friulian of Friuli. Rhaeto-Romance languages can be classified as Gallo-Romance, or as an independent branch of the Western Romance languages. The
Occitano-Romance languages of Southern France and East Iberia, includes Occitan and Catalan. Occitano-Romance languages can be classified as Gallo-Romance, Iberian-Romance, or as an independent branch of the Western Romance languages.
The Gallo-Italic languages. They include Piedmontese, Ligurian, Lombard, Emiliano-Romagnolo, Gallo-Italic of Sicily and Gallo-Italic of Basilicata. Gallo-Italic languages can be classified as Gallo-Romance or as Northern Italian dialects. The Venetian language is sometimes included in Gallo-Italic, but it has several characteristics that set it apart from it. 
The Oïl languages, Arpitan and Rhaeto-Romance languages are sometimes called
^ a b David Dalby, 1999/2000,
The Linguasphere register of the world's languages and speech communities. Observatoire Linguistique, Linguasphere Press. Volume 2. Oxford.    Archived 2014-08-27 at the Wayback Machine
"Romance languages". Encyclopædia Britannica . Retrieved 2017. ... if the Romance languages are compared with Latin, it is seen that by most measures Sardinian and Italian are least differentiated and French most
La pronuncia italiana (Italian). treccani.it
^ Lorenzo Renzi, Nuova introduzione alla filologia romanza, Bologna, Il Mulino, 1994, p. 176 «I dialetti settentrionali formano un blocco abbastanza compatto con molti tratti comuni che li accostano, oltre che tra loro, qualche volta anche alla parlate cosiddette ladine e alle lingue galloromanze [...] Alcuni fenomeni morfologici innovativi sono pure abbastanza largamente comuni, come la doppia serie pronominale soggetto (non sempre in tutte le persone)[...] Ma più spesso il veneto si distacca dal gruppo, lasciando così da una parte tutti gli altri dialetti, detti gallo-italici.»