The flexible board (9116F) is adjustable for any concave or convex shapes. The board has yellow dot marks that indicate the straight shape and red dot marks indicating the maximum shape. The rigid board (9116R) comes without the flex bar but the same handle enables a flexible and ergonomic grip. The low profile and the durable construction give a very good control over the sanding process. The performance is excellent in combination with Mirka Net stripes.