Related Clues:

• Loneliest number
• Air Force follower
• Solitaire quorum
• 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