Clue by four. Rhymes with two by four. Thusly the Clue by Four is a board with which one whacks people over the head. The purpose of this whacking is so that the whackee may perhaps be inspired to get a clue. The idea of the Clue by Four predates the invention of the two by four. It can be traced back to cave men and big clubs and all that nonsense.
The clue in Clue by Four stands for what is being doled out. The by four stands for the width of the board. Note that due to the milling process, most Clue by Fours are only 3 3/4 inches wide, and not four. Clue by Three and Three Quarters just doesn't have the same ring to it.
The Clue by Four doesn't necessarily have to be a two by four. There's no official body that defines just how wide a clue is. However a two by four is probably the opitmal board of choice. If a one by four is used, it will be too thin and snap when someone is 'given a clue'. On the other hand, a four by four is a bit too large to get a good grip around without using both hands. Thus the two by four is just right.
To determine the optimal length for your Clue by Four, you can use a trick used for picking the correct length of baseball bat. Hold a good sized Clue by Four by the end with one hand. Extend the arm holding the board out straight and let the Clue by Four hang down vertically. Then try to rotate it upwards so that the board is pointing away from you and parallel to the ground. If you can't perform this action, then try a shorter Clue by Four. If it's way too easy, then try a longer one. Somewhere in the three to four foot department will allow for good reach with decent control.
When you have picked out your Clue by Four, you might want to write 'Clue by Four' on it in large letters. This lets others know just why exactly you're brandishing a large piece of wood in their general direction.
While most Clue by Fours are metaphorical, I have seen one in the flesh (or should that be in the wood). The Beta Nu chapter of Theta Chi at Case Western Reserve University has a Clue by Four that resides in "The Barn". I have borrowed it on more than one occasion and taken it with me to classes with especially clueless professors. Unfortunately this seems to have no effect on the professor, but is good for a laugh. The Theta Chi Clue by Four is a three foot long two by four, painted white, with CLUE X 4 written on both sides in black marker. It also has a head on it, that is made of a piece of wood with somewhat pointed ends that is screwed into the top. This makes it that much more effective.