I just recently found out that only a taker and a maker can be matched to complete a trade.

I used to think that two takers could also be matched.
Suddenly it all makes sense, and I feel pretty dumb.

For example, one taker buy and one taker sell—
in reality, these two can't complete a trade.
This rule at exchanges requires a market maker to be involved.
It also helps me better understand OI, as well as the logic behind price increases and decreases.