chữ kép

Etymology
🇰🇲.

Noun

 * 1)  compound phonograms, a category of  where each character is composed of two components that are both phonetics; these characters were more popular in the earlier Nôm texts, representing words with initial cluster or presyllable, but their usage persists to some extent even after phonological changes had rendered the phonetics obscure
 * 2) * Chỉ nam ngọc âm giải nghĩa (指南玉音解義)