Every year on October 22nd, enthusiasts around the world celebrate Caps Lock Day. A day where capitalisation is celebrated in all its glory. But did you know that using capitalisation in your email subject lines can also impact your open rates? We analysed eight million delivered emails and made an interesting discovery.

A fascination with capital letters?

The origins of Caps Lock Day weren’t due to its creator’s fondness for capitalisation. On the contrary, he (Derek Arnold) was fed up with it. In an effort to curb excessive capitalisation, he used Caps Lock Day to raise awareness about how often people pressed the key.

The reason Caps Lock (or rather, excessive capitalisation) is used is, of course, to draw attention to the word and, consequently, the rest of your message. In cases where boldface or a different font size isn’t an option, capitalisation or symbols are often the means to grab attention in email marketing. To investigate whether we can actually attract attention by using capitalisation, we conducted research into the effect of a word in Caps Lock at the beginning of an email subject line.

Whether or not to start with capital letters

The research we conducted was quite simple: What effect does capitalising a subject line have on your email opens? We analysed a total of eight million delivered emails across nearly 200 campaigns. To determine whether capitalisation has an effect, we divided the subject lines into two categories:

  • Group ‘No Cap(s)’: the subject lines without the use of three or more capital letters at the beginning of the sentence.
  • Group ‘CAPS’: The subject lines with three or more capital letters at the beginning.

The “No Caps” group, meaning those without Caps Lock, contains 4.2 million delivered emails. The “CAPS” group contains 4.1 million, a minimal difference. It’s worth noting that the selection was completely random. In our data, we split the emails based on Caps Lock and then selected the first ~4 million.

What turns out? The “No Cap(s)” group generated 549,976 opens, while the “CAPS” group saw 771,032 emails opened.

Group ‘No Cap(s)’Group ‘CAPS’
Delivered emails4.154.1644.114.082
Opened emails549.976771.032
Conversion rate13,23%18,72%
The ‘conversion’ percentage of emails delivered.

Against expectations

When we started this little experiment, the assumption (or hypothesis) was that the difference would be minimal and insignificant, or, in the worst case, that the subject lines in the “No Cap(s)” group would convert better. Who wants all that yelling in the inbox?

However, the results contradict this, and the results as described above are significant enough ( -21.23) to conclude that capitalisation at the beginning of the subject line has a positive effect on your open rate.

Which words are often capitalised?

Of course, it’s interesting to know which words are capitalised. Because our selections consist of English-language campaigns, we can only provide English terms as examples, but even these give an idea of ​​what’s effective. We found nine words that achieved higher conversions than the average ( >18.72% ):

TERM in Caps LockConversion rate
ACTION29,8%
COMING SOON38,5%
DOWNLOAD24,6%
EXCLUSIVE30,6%
IMPORTANT24,6%
INVITATION23,9%
REMINDER30,1%
SAVE20,9%
WIN20,1%
An overview of the terms in the ‘CAPS’ selection that scored higher than the average.

Should you now use Caps Lock in your subject line?

Based on these results, to say that (excessive) capitalisation in subject lines should become the norm is a bit much. Still, it’s wise to experiment with the effect of Caps Lock on your email marketing. Start small. Set up an A/B test where you only capitalise the first word of your subject line—and then see how that affects your open rate.

An example of how to use Caps Lock in an A/B test in Spotler Mail+

Also, keep in mind that results may vary depending on your target audience and the type of content in your email. Experiment and discover what works best for your campaigns.

Happy CAPS LOCK DAY!