What is the “sinosphere”? It refers to “East Asian cultural sphere,” or 漢字文化圏, in Japanese and Chinese. Many if not most people, including Japanese, hardly know that Vietnam was part of the “sinosphere.” That is, the countries and regions where Chinese characters are or have been used to write their language. Well, everyone knows that contemporary Chinese, Japanese and Korean (fading out) use Chinese characters, but did you know what languages are shown here?
They are Cantonese, Taiwanese and Vietnamese. The last one, called Chu Nom (Chữ Nôm 喃), is Vietnamese written in Chinese characters or characters created on Chinese model. Most people are quite surprised to learn that for about 1000 years Vietnam was part of the Sinosphere.
For Japanese and Chinese speakers like myself, it helps a lot when learning new Vietnamese words to know the underlying characters, like 政治 (J:seiji, V: @chính trị), which are cognates and are pronounced similarly. Though the Chu Nom script was mostly abandoned in the 1920s, you still see Chu Nom on signs here and there. Here is a wall with Chu Nom writing right outside my hotel window. And of course you will see characters in the many Buddhist temples.
There is even a website that enables you to input Chu Nom at Chu Nom online editor.