Abstract: A masonry line block is adapted for use in combination with a vertical corner pole during construction of a structure. The structure has an exterior surface and at least one surface projecting from the exterior surface. The line block engages the corner pole at one end of the structure and cooperates with a line holder located at an opposite end of the structure to carry a tensioned guide line therebetween. The guide line indicates proper placement and alignment of building units used to form the structure. The line block includes a mounting member having first and second, opposing pole-engaging shoulders. The shoulders are adapted for selectively engaging the corner pole to hold the line block in one of first and second alignment positions relative to the structure. An alignment member is formed with the mounting member and extends perpendicularly outwardly from the pole-engaging shoulders.