Just as the US dollar, Canadian dollar, Hong Kong dollar (Hong Kong currency), New Taiwan dollar, Australian dollar, Singapore dollar, Argentine peso and Chilean peso all use the $ symbol under the Spanish silver dollar system, Brazil and former Portugal all use the two-line version, which is just a variant of an abbreviation.
As for why it is $ instead of D plus a horizontal, it is because it is actually a variant of PS (peso peso). Similarly, the pound is a variant of LS (Librashiling) and the Euro is a variant of the initial e of Euro.
Yen "y" Source: At the end of the curtain, the British and Americans called Yan instead of the Japanese Roman word en. In order to cater to the habit of two lines of dollars, we added two lines after the initial letter Y.
Source of RMB "Y": The symbol of RMB is the first letter Y of the pinyin Yuán with RMB unit "Yuan", plus two horizontal lines. .
The currency symbol of the Japanese yen is "?" , the same as the symbol of RMB (about how to distinguish these two monetary units, what is the symbol of RMB if these two currencies are to appear at the same time?) CNY before, or in Japanese yen? Put the word JPY in front of it to distinguish it.