The requirement for an opening bid of 2C is 20 or more points.
Some players use 22 or more points or 8.5 tricks as their requirement.
There are three schemes of responses.
1. A response of 2D is 'waiting' for opener to describe the hand further.
A response of 2H is negative (less than 2 queens); opener must immediately
place the contract.
Other responses are natural with 8+ points. A response of 2N shows hearts.
2. A response of 2D is 'waiting' for opener to describe the hand further.
A response of 2H,2S shows 8+ points and 5+ cards headed by two of top three
honours.
Other responses are natural with 8+ points.
After opener rebids over a 2D response, the cheaper minor is a second
negative response.
This gives the strong hand two bids to describe the hand and, hopefully, land in
a better contract.
3. Responder shows controls - counting Ace as 2 and King as 1.