Related Clues:

• Something for the books
• The first cardinal?
• It has bark but no bite
• Liftoff preceder
• Word on a dollar
• Middle X or O?
• Hierarchical chart
• Squirrels' hangout
• Swing support, perhaps
• It's stuffed with wood
• Hydrogen's number
• Area-code preceder
• Score on an aced hole
• 'Tic -- Dough'
• Part of binary code