Just an editing note to anyone using "One Zentangle A Day"
Day 16 - the examples shown - the last one (green) is NOT an example of Achromatic Scale. Achromatic is when only black, white and grey are used and this example uses shading of greens. That would make this image Monochromatic. Same error on Day 20.
Day 19 -The step-out directions for Cubine are incorrect. The second step is a mirror image of what it should be. Use this page instead of you book to make this tangle: http://tanglepatterns.com/2010/05/how-to-draw-cubine.html
Those are the errors I have found so far. I'll add more if/when I find them.