It won't say 1/8 on the 1/8th line, but 1/8 is half of 1/4. So the little line between 1/4" lines is 1/8. Since 4/8 = 1/2, then 3/8 is 1/8 less. So 3 3/8 is the little line right before the 3 1/2 is 3 3/8. Make sure when you're measuring that your ruler is Straight with one edge and the bottom edge. Hope that's clear.