 Diacritic characters used for uppercase vowels are shown boxed, and those used for lowercase vowels are shown unboxed.The VNI encoding for use on DOS does not use separate characters for diacritics, instead replacing certain ASCII punctuation characters with tone-marked uppercase letters (compare ISO 646).The use of Vietnamese Quoted-Readable (VIQR), a convention for writing in Vietnamese using ASCII characters, began during the Vietnam War, when typewriters were the main tool for word processing.