Measuring Triangle for Shellfish Print this template and make copies. You might want to use waterproof paper or laminate the measuring gauge. To be sure it is the right size, check to see that the vertical axis is exactly 140-mm long and the top line is 100-mm long. If it does not print to exactly this size, you will need to redraw it. Each horizontal line increases by 5-mm from 5-mm to 100-mm. To draw it again, for example on white Perspex, the horizontal lines are spaced seven millimetres apart. First draw the vertical 140-mm line then the top 100-mm line to form the basic triangle. To size shellfish, place the shell on the triangle with the opening up, and the hinged area down. Move the shell up or down, keeping the longest dimension of the shell parallel with the horizontal lines. When the ends of the shell just touch the edges of the triangle (look straight down to avoid parallax) record it's longest dimension as the number on the line on the top of the zone where the shell overlaps the edge.