Genuine John Deere mower blades are designed to exact specifications to achieve the best performance and cut quality. John Deere use high quality steel that stays sharper for longer. A flat mounting surface reduces blade stress for long life and excellent cut quality. The properly sized center hole provides for less vibration and increased machine life. When you use these M145719 mulching blades with a BM21890 mulch kit, you get perfectly sized clippings that return nutrients to the lawn and reduces the need to dispose them.