A cutting mat is NOT a ruler and should not be used for precise measuring.

• Why? Because of the coefficient of expansion. Basically mats are made of a soft plastic, so they have a
  greater chance of expanding and contracting in different weather. Rulers are always made of acrylic which has a much, much smaller coefficient of expansion. As a matter of fact the coefficient of expansion on rulers is so small as to be practically non-existent for a quilters purpose.
• You should be using your ruler to measure precisely, and should be using the same ruler throughout the
  project which is something every top quilt instructor teaches.
• Mats should be used only for general measuring and for cutting. They should never be used for precise
  measurements.