Related Clues:

• Adding column
• A quarter of four?
• Loneliest number
• Air Force follower
• Till slot
• Binary code elements
• Decimal column
• Solitaire quorum
• With 51-Down, a common tennis score
• Ace, sometimes
• Something for the books
• The first cardinal?
• Some binary digits
• Liftoff preceder
• Word on a dollar