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Write a subject line test pair that differs in only one thing

  • For Marketing and content
  • You practice Setting limits and rules
  • Time about 5 minutes
  • Level Beginner

About this exercise

You're setting up an A/B test for the Fernhill Planner newsletter. Write a prompt for exactly three plain-text lines: "A:" with a subject of 7 words or fewer that contains the word tips, "B:" with a subject identical to A except that the number 3 is added at the very start (lowercase the old first letter if needed), and "Changed:" naming the single thing that differs. Neither subject may use an exclamation mark or the word free.

Common mistakes when you ask AI for this

  • Changing two things. The AI rewrites B with a different tone and length. Try: Say: "B is A with only the number 3 added at the start."
  • Using banned words. The AI writes 'Free tips!'. Try: Say: "No exclamation mark and no word free."
  • Missing labels. The AI lists options with explanations. Try: Show the three labels: A:, B:, Changed:
  • Letting markdown through. AI adds bold text and headings by default. Try: Add: "Plain text only, no bold, no headings."
  • Trusting the AI to count words. AI is bad at counting and often runs past a word limit. Try: Cap each part (for example "each line under 15 words") so the total lands well under the limit.

What a strong prompt includes

  • Gives the three labels
  • Requires one difference
  • Names the change
  • Bans words and punctuation
  • Caps length

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