These centers will improve your drilling accuracy. Zinc plated steel centers perform three basic alignments. For dowelling, the centers will make correctly positioned marks for precise drilling of your second set of dowel holes. For properly positioning chair rungs or other spindle work, slip a center over the end of your tenon and use pinpoint the location for boring. To find the center of a dowel slip a center over the end and tap lightly. An internal point will mark the exact center for lathe work. The 8 piece set contains 2 of each size.