Swing Weight
A measure of how heavy a club feels as it swings - the balance of head weight against grip weight, expressed on a scale like C9, D2, D4.
Swing weight is not the total weight of the club. It is a measure of how the weight is distributed - specifically how heavy the head feels relative to the grip end when you swing. It is expressed on a lettered scale (C, D, E) with numbers from 0 to 9, where D2 is a common men's stock value and higher numbers feel more head-heavy.
Two clubs can weigh exactly the same on a scale yet feel completely different in the swing because of where that weight sits. Add weight to the head or lengthen the shaft and swing weight rises; add weight to the grip end and it falls. This is why grip changes and shaft cuts both alter how a club feels even when the loft and lie are untouched.
The right swing weight lets you feel where the clubhead is throughout the swing without the club feeling either sluggish or whippy. Players who like to feel the head tend to prefer higher swing weights; players who fight a hook or want more speed often go lighter.
Swing weight matters most for consistency across the set. A well-built set keeps swing weight uniform club to club, so every iron feels like a member of the same family rather than a collection of mismatched tools.