Face Side and Face Edge (Datum) Marks
Right: Oops! Why don’t the grooves line up?
The worker failed to keep the fence of the plough (or router) against the face-edge.
Right: 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.
Right: They can prevent the kind of error that lead to the 'shovetail' joint on the left. This can happen when novice makers forget to align the face-sides so that they face each other.
Below: 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.
Right: 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 90deg.
You could check this with a sliding bevel set to an angle other than 90deg.
Above: 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.
A Woodworker’s Notebook