Opening 1 Diamond shows unbalanced hand
Requirements to open 1♦
Our 1♦ opening bid is restricted (slightly). When we open 1♦ we are stating that we have 4 or more diamonds and a singleton or void in a side-suit. If we don't have a singleton or void, then we will have at least 6 diamonds. This considers a hand with 2362 or 2272 shape to be unbalanced. If we don't have this requirement, we will open the hand with our Taco 1C. This means that we might open 1♣ with longer diamonds than clubs (e.g. 3352 shape hand) still opens 1♣ with only 2 clubs and 5 diamonds.
Responses to 1♦ We respond naturally to the 1♦ opening bid. Keep in mind that the opener is showing an unbalanced hand, so your long suit might (and probably will) be their shortness. However, this extra information transmitted in the first bid is powerful. Also, if a fit is found the known shortness elsewhere will greatly aid in game invitations
Read Blog artical Unbalanced Diamond for more details and advanced responses and rebids.