The langues d'oïl are a dialect continuum that includes standard French and its closest autochthonous relatives historically spoken in the northern half of France, southern Belgium, and the Channel Islands. They belong to the larger category of Gallo-Romance languages, which also include the historical languages of east-central France and western Switzerland, southern France, portions of northern Italy, the Val d'Aran in Spain, and under certain acceptations those of Catalonia.

In linguistics, mutual intelligibility is a relationship between languages or dialects in which speakers of different but related varieties can readily understand each other without prior familiarity or special effort. It is sometimes used as an important criterion for distinguishing languages from dialects, although sociolinguistic factors are often also used.