Barred D Đ đ is used in Vietnamese Orthography
to express a sound English speakers hear as / d / but linguists say is / ɗ /
whereas D in modern Vietnamese is the sound /z/ or /j/ depends on which dialect you use.
Why is this so?
I'm not sure if it was brilliant design or someone trying too hard to please a variety of users, European and Vietnamese.
The Jesuit scholar and priest who created the first Portuguese Latin Vietnamese Dictionary published in 1651 Alexander Rhodes was one of those geniuses who can learn new languages in weeks or months and was fluent in French Portuguese Spanish Greek Latin Chinese Vietnamese and other languages too.
In the 17th century Vietnamese had a "Dh" sound now written as D the implosive sound now written with a barred D a T and a 'th" which was aspirated.
The "dh" written as D later split into Z and J sounds and was used in Spanish and Portuguese though its now written z in Spanish and never printed or written as any form of D with a bar or stroke.
The distinction is very important in Vietnamese not so much in Romance languages.
He needed a form that could be both written and printed.
No record that I know of has survived explaining why barred D was chosen for D and D as a sound probably already shifting in popular speech?
Perhaps the simplest explanation is that barred D was available as part of a printers font set because it was used as a form of shorthand by scribes to shorten long Latin words in texts?
Also new fonts being cast in metal ... adding one stroke was a simple change and z and j were still not fully established in font sets with printers still using y or i for j.
I think he opted for the simplest solution cos he had to please the printers, native Vietnamese scholars and converts with mixed feelings about European alphabets, the Vatican, various missionaries and merchant captains, his fellow Jesuits, or possibly after working on diacritics to express six different tones only having to change one consonant symbol was a sheer relief?
And despite overhand doing a great job on adjusting the Roman ABc to write Vietnamese people still argued for several centuries over this ABC orthography versus using Chinese characters to use write an Austroasiatic language top heavy with loan words from several Chinese dialects!
Why no one thought to try to bridge the factional differences by creating a French Vietnamese Chinese character dictionary before the 20th century ... oh politics! If the French had been less ...arrogant and thought to undermine the conservative mandarins by trying to make learning Chinese Characters easier because there were printed reference texts available ... would Vietnamese history have differed?
Barred D a letter with one stroke making it different to D.
Design can have a political nationalistic context especially when your design aim is to get people to learn and switch forcibly or by choice from one script to another or to acquire literacy more easily in a national or regional language competing with an older script in that or another prestige language.
Something to consider?
As a designer though before politics or religion or culture the aim should be a script and letter shapes that are easy to learn yet also elegant and shapely pleasing to look at and read yet accurately represent sounds and can be printed and also hand written or brushed!