You have your ruler. A blank piece of paper. Pencil in hand. You go to draw a lateral line and instead one gets a leaning tower of lead. Here's the secret to penning the perfect straight line.
Step 1.Place pencil down on paper to begin your first end point. Make sure pencil is sharp. Dull pencils thicken lines.
Step 2. Align ruler to pencil. Your ruler is just a guide. The pencil draws the line.
Step 3. Holding ruler steady, draw line from start point to end point. Never retrace line, for this will thicken the line and throw off measurement.