Points to note:
A Dontya alphabet (ন, দ, ত etc) create a compound with only a Donitya alphabet (ন, দ, ত, ধ)
A Murdhonya alphabet (ণ, ড, ট etc) create a compound with only a Murdhonya alphabet (ণ, ড, ট, ঢ)
The first alphabet is almost recognizable in the compound letters
Dontya Dho (ধ) when added as the second character, it retains its triangle but gets a hook on the back