For example:
- て+つな=たづな(手綱)
- め+ふた=まぶた(瞼・目蓋)
- かね+つち=かなづち(金槌)
The only thing I can see for sure is that the second word becomes voiced, but that's more of an after-the-fact thing than a rule that dictates when the sound actually changes from e to a.
Is there a rule or pattern to it?