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You can't breathe with the diaphragm when singing. The diaphragm is an involuntary muscle. As it contracts, it sucks air into the lungs; as it relaxes, the air in the lungs gets expelled. (This is an oversimplification of things.) What you may be referring to is using the muscles of support. The muscles of support are those that are attached to the ribs: the abdomen, intercostals, quadratus lombordum (sp?) and the lattisimus dorsi. These muscles help to keep the rib cage expanded and help prevent your diaphragm from wanting to come up quickly and expel all of your air out at once. It's hard to put into words whether or not you're supporting correctly because everything is interconnected -- the support has to be balanced with the laryngeal muscles. In general, if you're not constantly running out of air, you can say you're doing some kind of supporting.