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The Language of Subject Lines: Simple or Sophisticated?

When it comes to writing Subject Lines, a lot of marketers follow the KISS approach,“Keep it simple, stupid.”

But Alchemy Worx research based on Subject Line Pro has revealed that relying too heavily on simple language could be the reason why many recipients don’t read your emails.

Using Subject Line Pro to test on virtual recipients instead of real-life subscribers, we discovered that the greater the complexity of language used in Subject Lines, the better the click and open rates.

Although the results may sound counterintuitive, our approach to the study was anything but.


We used two methodologies for our study.

The first was the Coleman–Liau index. It relies on the number of characters in a word instead of syllables.

The second was the Automatic Readability index. Similar to Coleman-Liau, it uses the number of characters to gauge the understandability of text. 

The text was then classified according to the standard US grade system, ranging from understandable by a child in kindergarten up to the level of an undergraduate university student. 

Examples of subject lines:

Grade 2-3

  • Rewards Coupons, Fri. and Sat.
  • Big flight savings
  • Don’t miss these awesome deals

Grade 6-7

  • You qualify! Because you’re an email subscriber: awesome savings in top destinations
  • Tired of always looking exhausted?
  • Order gift cards fee-free: Perfect for graduates


  • Easy, flameless, effective. Cute odor-neutralizing Fragrance Spheres. Just $5.49
  • Designs with Character (Literally!)
  • Budget-Friendly Swimsuits, Embarrassing Prom Moments, and More

What the study found

Our study analyzed 675,000 Subject Lines and the results of 41 billion sent emails. All Subject Lines in the database were categorized according to understandability using the two chosen methodologies, before determining whether the language complexity had any impact on open rate, clickthrough rate or click-to-open rate. 

The grey bars in the charts below represent how many emails of each type were sent.

Best practice guidelines for Subject Lines typically advise marketers to keep Subject Lines as simple as possible. Therefore, many emails feature Subject Lines with an understandability level targeting people aged 9-14.

However, as the graphs show, the average open, click and click-to-open rates all tend to improve as the language in the Subject Line becomes more complex. In fact, the best performing Subject Lines were those with the vocabulary of a 16-to-18-year-old.

The take-away

Not only does this study suggest that complex language leads to greater email engagement, but it also demonstrates the value of testing and optimization.

It’s easy for marketers to assume that they know what language is most appealing to their customers. But to get the best results from email, it’s worth making use of tools like Subject Line Pro to test and optimize before sending email. Doing so will save time and resources, as well as boost engagement.


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We are all for optimizing subject lines – we created – but we also like to help clients explore more visual opportunities in the inbox that can help clicks

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