Abstract: A slide rule guide enables immediate selection of the requisite individual different diameter, different length sections or segments of a multiple segment tapered fly fishing leader. An envelope forming side panels slidably receives a planar slide. A pair of elongated windows in line and at right angles to the direction of slide movement simultaneously display common columnar data on the face of the slide; the overall length of the leader, the tippet size, and in order from the butt end of the leader to the tippet end, the length and diameter of each leader segment or section.