Related Clues:

• Subjects for Barron's: Abbr.
• Yellow Pages listings: Abbr.
• See 55-Across
• ___ qua non
• Trig. function
• Kind of curve, in math
• Digression
• Lettuce
• Comic Bill, familiarly
• Trig function
• Beloved comic's nickname
• Kind of curve
• Kind of wave
• Comedian Bill, to friends
• Ratio of AB to BC, say