This article is from the Woodworking FAQ Collection 1, by multiple authors.
When you steam bend apiece of wood, and clamp it to a shape, you wait
24 hours for it to cool thoroughly. When you take it off the jig, that
wood will spring back somewhat. How much depends upon the grain and
the type of wood - it's hard to say. If your stock has a natural curvature
in the required direction to start with (I try to take advantage of this
whenever possible), you will get less springback.
So if you have to get a certain curvature to the final product, make
your jig with greater curvature.
 
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