Ivory Soap ... 99 44/100% Pure" (The Literary Digest, Jan 18 1919)
The pattern
This still is filed under Campaign line as series.
Define
One line that can be replayed across executions so the campaign is a family, not a one-off.
Mechanism
Repeatable frame. Each ad is a verse. The line is the chorus. Recognition compounds because the structure is stable.
Verdict
We will run more than one execution and need them to add up in memory.
Why it works
A precise, awkward number does work a round one cannot: 99 44/100 reads as the output of an assay rather than a claim, and the specificity is itself the proof. The body copy never argues chemistry, it sells sensation, mild white bubbling lather and the feeling of cool cleanliness, so the reader is given something to want and a number to justify wanting it with. The signature lock-up compresses brand name, pack, the floating claim and the purity figure into one line, which is why the same line could run unchanged for decades and still carry the whole argument.
Steal this when
When a quality claim needs credibility and you have no testimonial: replace the round number with the exact measured one, put it in the signature line, and keep the body copy sensory. Use when the brand is already known and the job is reinforcement rather than persuasion.