A device for measuring linear distance comprises a longitudinal ruler body bearing a major graduation, which is constituted of a plurality of evenly spaced marking lines, around one of the long edges on the top surface and a basic measuring unit is defined by the distance in between two consecutive marking lines; a moveable member mounted and slidable on the top surface of the ruler body that the moveable member has a transparent section partially covering the major graduation; a slanted line, having a first tip and a second tip, located on the transparent section with the first tip capable of intersecting the marking line and spanning across the transparent section by the width of n unit of basic measuring unit that length of the slanted line is equally divided by a plurality of sub-marking lines to form a minor graduation composed of multiple sub-measuring units along the length of the slanted line; a pair of jaws, namely a first jaw and a second jaw, perpendicularly extending out from the ruler body and the moveable member respectively; and a transparent structure, attached on the moveable member and capable of traversing back and forth in relative to the transparent section, which has a latitudinal reading line marked in the middle of two guide lines and defining width of n or 2n units of basic measuring unit; wherein the reading line has sufficient length to intersect with the slanted line and n is an integer