Why can Eastern Armenian and Western Armenian be considered different languages?

Eastern and Western Armenian are two distinct literary varieties of the modern Armenian language. While they share a largely common vocabulary, they are often considered separate languages due to significant divergences in phonology and grammar.1

The primary differences include:

• Consonant Shifts: The two varieties exhibit different developments of the Old Armenian plosive system. For example, in Eastern Armenian, the pronunciation of certain consonants remains closer to traditional forms, whereas in Western Armenian, these sounds have undergone shifts, such as the voicing of previously voiceless stops.2
• Standardization: These varieties emerged in the 19th century as modern standard languages. Western Armenian was based on the dialect of the Armenian community in Istanbul, while Eastern Armenian was based on the dialects of Yerevan and Tbilisi.1
• Geographic Distribution: Eastern Armenian is primarily spoken in Armenia, Azerbaijan, Georgia, and Iran, while Western Armenian is used by communities in Turkey, various Arab countries, and the Armenian diaspora in Europe and the United States.13