many a times the mother tongue is not accurately known
nor learnt by the native speaker.
those who 'learn' other languages are much more systematic
as they learn the 'correct' form from authoritative sources like
grammar books, dictionaries, teachers, etc
bob's writeup is the most accurate explanation for the two
letters that i have come across in about 4 decades
i am curious whether these two have a place in the devanaagarii
script or are 'external' to devanaagarii (with some sanskrit
examples using the two letters)
in hindi, the two are used with great frequency
(just recording here that ड़ ड and ङ look very similar but each
is a totally separate stand-alone consonant with its own totally
separate sound. Also, ड़ cannot and should not be created on
a computer by assuming that ड + ़ = ड़)
..peekay