Finding the centre of a board

There are many tricks for finding the middle of a board without doing any math. The photo shows a centre marking jig, two dowels in a board with a pencil dead centre, rotate the board until the dowels rest on the two sides of the workpiece, then simply slide the jig along and you have a centre line. \012\012Putting a tape measures diagonally across a workpiece so as to have some even number of inches, like 10 and 20 lined up with the two edges, gives the centre at 15. There are even centering rulers available that measure out from the middle of the ruler, and you simply move it back and fourth until you get the same reading on both sides.\012\012I got my centre ruller from Lee Valley Tools.