Grading criteria:

Master (very rare to get): straight, very regular and tight grained, uniform color

AAA (good tap tone, very stiff):
mostly straight, tight grained, tending to somewhat more open grain spacing on the outer edge, slight color variation possible

AA (good tap tone):
less tight to more open grain spacing, slight color variation and/or curved or wavy grain possible

A (good tap tone, use on tinted tops):
wider grain spacing, slight color variation and/or curved or wavy grain possible