straightedge

from WordNet (r) 3.0 (2006)

straightedge
    n 1: hand tool consisting of a flat rigid rectangular bar (metal
         or wood) that can be used to draw straight lines (or test
         their straightness)

from The Collaborative International Dictionary of English v.0.48

Straightedge \Straight"edge`\, n.
   A board, or piece of wood or metal, having one edge perfectly
   straight, -- used to ascertain whether a line is straight or
   a surface even, and for drawing straight lines.
   [1913 Webster]