A Woodworker’s Notebook
Jeff Gorman
Face Side and Face Edge Marks (Datum)
The looped face-side mark shoots off the face and joins the face-edge mark. As well as indicating datum surfaces and showing the mating surfaces for joints, the fs & fe serve as reference faces for fences of machine and hand tools
Oops! Why don’t the grooves line up?
The worker failed to keep the fence of the plough (or router) against the face-edge.
As a case where the face-sides are not co-planar. It will not unduly matter if some frame joint tenons and mortises are not quite dead centre, providing that gauging, etc is always done from the datum faces.
They can prevent the kind of error that lead to this 'shovetail' joint. This can happen when novice makers forget to align the face-sides so that they face each other.
Illustrating how a try square should always rest against a datum face.
Providing that the datum faces and the stock of the square are true, the lines should join up, even if the square is not set at a true 90°.
You could check this with a sliding bevel set to an angle other than 90°.

These long-standing traditional marks relate to hand woodworking. While the marking of datum faces on one-off machine work is still a desirable practice, it is not always possible to offer a datum face to a machine fence. In this case very accurate thicknessing will avoid the kind of error shown in the framing joint diagram.

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