Related Clues:

• With 51-Down, a common tennis score
• Ace, sometimes
• Something for the books
• The first cardinal?
• Liftoff preceder
• Word on a dollar
• Hydrogen's number
• Area-code preceder
• Score on an aced hole
• Part of binary code
• Hour after noon
• Count starter
• Square __
• Countdown end
• White Monopoly bill