What are the two written varieties of Modern Armenian?

Modern Armenian, also known as Ashkharhabar, consists of two distinct written varieties:

• Western Armenian (Arewmtahayerēn): Historically referred to as the "Armenian of Turkey," this variety is based on the dialect of the Armenian community in Istanbul. It is primarily used by Armenians in Turkey, various Arab countries, and among emigrant communities in Europe and the United States.12
• Eastern Armenian (Arewelahayerēn): Formerly known as the "Armenian of Russia," this variety is based on the dialects of Yerevan and Tbilisi. It is the prevalent variety in Armenia, Azerbaijan, Georgia, and Iran.12

While these two varieties share much of their vocabulary, they exhibit significant differences in grammar and pronunciation, to the extent that they are sometimes considered two different languages.2