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dbOCD takes an input of 2 or more vertices lying on the same edge loop and aligns the rest of the vertices on that edge loop to form a smooth curve or arc. The curve type parameter determines whether the input vertices define points on an arc, edit points, or control vertices. The arc option can only take exactly 3 vertices as an input, if more or less than 3 vertices are selected it defaults to an edit point curve. The closest point option requires the plugin closestPointOnCurve, which comes with Maya 7's Bonus Tools (and possibly other versions, I haven't checked).