sentences:  hand, wrist, or scalp veins (hence name "scalp vein" set).Butterflies are commonly available in 18-27 gauge bore, 21G and 23G being most popular.In phlebotomy, there is widespread avoidance of 25G and 27G butterflies based on belief that such small-bore needles hemolyze and/or clot blood samples and hence invalidate blood tests.