I'm curious as to the history of Thai tone marks. Originally Thai has just
two tone marks, a vertical bar (mai ek, literally 'mark one') and a cross
(+) (mai tho, literally 'mark 2'). When were the other two, mai tri
(literally 'mark 3') and mai chattawa (literally 'mark 4'), added?
(Obviously they won't have been used until the orignal tones had split.)
Mai tri seems chiefly to be used in English and Chinese loanwords. Why
does mai tho now look like a European '2'? (It bears little obvious
resemblance to the Thai digit '2'). It is notable that the present day mai
chattawa is now a cross (+), like the original mai tho.