Our height charts are printed on the highest grade CARB compliant Baltic Birch plywood. Birch is a highly renewable resource and the natural wood background lends a unique look to each chart. No two charts are exactly the same, as some have more noticeable wood grain than others while others may have naturally occurring bird's eye knots. We find these naturally occurring marks give each of our handcrafted charts a unique character all its own. The inks used in the printing of each growth chart are no-voc inks, called UV curable inks. They are solvent free and will not fade over time. Printed directly onto the wood, bonding with and staining the wood fibres. We do not use toxic polyurethanes as they can gas off in your home for up to five years. Use any pen, pencil or sharpie, and begin marking moments in time! We hand sand our products, making the edges super soft. OUR CHARTS ARE GUARANTEED FOR A LIFETIME! BUY WITH CONFIDENCE! Meticulously made in the USA. Because years filled with growing children are years to remember! Happy Measuring! As always, let us know if you have any questions. We’d be happy to hear from you!