Opening Bid: Two Clubs
This is a frequently used bid in all strong club systems... It shows an opening hand with a club suit. Players often modify it with different agreements, but the most common way of playing it is to have 11-16 HCP, and at least a 5-card club suit. Some players prefer to promise a 6-card suit but that agreement restricts the use of the bid. The opener may have a 4-card major.
Four Diamonds - Five Clubs
With four diamonds and five clubs, opener should start with a 1 call and then rebid 2.
Five Diamonds - Five Clubs
With five diamonds and five clubs, strong club systems use an opening bid of 2NT to show both minors.
Responding to an Opening Two Club Call
If responder uses the 2 asking bid and then rebids a major at the 3-level, the bid is natural with at least a 5-card suit and forcing.
Negative Double by Opener
This is about the only place I can think of where the opener gets to use a negative double in a competitive auction... Suppose the opener bids 2 and responder asks about a 4-card major by bidding 2. then, suppose the next hand overcalls with a suit, for example by bidding 2, and opener has a 4-card heart suit... Seems like a perfect spot for the opener to double.