I found this in one of my favorite grammar books ***:
The concessive relation is sometimes very similar to that expressed by co-ordinate clauses showing contrasts.
It gives these examples:
1. ALTHOUGH he was large, he was not strong. (Concession; subordinate clause)
2. He was large, BUT he was not strong. (Contrast; co-ordinate clauses)
That book says that co-ordinate clauses of contrast are connected by "but," "nevertheless," "however," "yet," "on the other hand," and "on the contrary":
a. The society is only four years old; however, it has already effected a number of reforms.
*****
Clauses of concession are subordinate (not co-ordinate):
3. Even if he is wealthy, he is not happy.
*** Reference: English Review Grammar by Walter Kay Smart.