Box end has built-in N-PowerFit Drive- Increased strength and reduces rounding or fastener damage by distributing stress through a gentle arc in the bore by enlarging the contact surface Box end flat ground so teeth reach edge of wrench vs beveled for 20% greater grip strength Open end has reinforced teeth to prevent cracking and breaking Machined to tightest dimensional tolerance for best fit in the industry Angle rise of 15 degree for box end Clear engraving- The beautifully-refined engravings are easy to see and increase the efficiency of work. Compact external diameter is among smallest in the world and ideal for tight spaces Set contents 10, 11, 12, 13, 14, 15, 16, 17, 18, 19mm