Hi teachers!
I was wondering if 'overtone' and 'undertone' were interchangeable.
For example in these two sentences:
There were politcal overtones to the point he was making.
The play does not have the political undertones of the novel.
I looked up both words in the dictionary and it says 'overtone' is usually used in the plural but says nothing in this regard for 'undertone'. So I also wonder if 'undertone' is a word often used in the plural.
When I looked up 'undertone', 'undercurrent' appeared as a synonym, but when I looked up the latter, I found that the two words were used quite in the same way.
E.g.
I detect an undercurrent of resentment towards the new proposals.
Could someone explain the differences among the three words?

Thanks in advance!