A hollow-body iron has a fully enclosed, welded head — typically a thin forged face fused onto a separate cast body — rather than the single-piece construction of a traditional iron. The empty internal cavity lets the face flex more freely at impact, like a small trampoline, which raises ball speed on long irons where players most need help; TaylorMade's P790 popularized the construction by welding a forged face as thin as 1.75mm onto a cast body and injecting SpeedFoam Air behind it to control the hollow chamber's sound and vibration. Because a fully hollow head can feel and sound harsh without that damping, most iron sets taper the construction — hollow through the long and mid irons, shifting back to solid cavity-back or forged short irons where feel and precision matter more than raw speed.