Email Subject Lines: How to Convert with the C.U.R.V.E. Method

Alex Williams

There are two ways you can look at the practice of writing email subject lines:

  • An unforgiving onslaught of repetition under duress that drains your creativity and energy
  • An exciting and rewarding challenge that’s an opportunity to improve marketing and communication skills

Those of us who fall into the latter category hope that someday the stars will align and we will write the perfect subject line: 100% open, 100% click through, and 100% conversion. After years of writing and testing subject lines for all ends of the business spectrum, we’ve developed a unique method of creating subject lines called C.U.R.V.E. This new method improves your ability to write and test email subject lines effectively.

Using C.U.R.V.E. to Create Email Subject Lines

There are five parts to the C.U.R.V.E. method:

  • Curiosity
  • Urgency
  • Relevancy
  • Value
  • Emotion

A good subject line should possess at least two of the five C.U.R.V.E. elements and at least one of those two elements needs to be Relevancy or Value. The goal of an email subject line is to get the recipient open your email. Email subject line best practices are all well and good, but the nature of subject lines requires flexibility. You’ll notice the C.U.R.V.E. method doesn’t offer a suggested character length, capitalization rules, spam words to avoid, or punctuation guidelines. Why? Because subject lines are the easiest thing to test in email marketing and what works today may never work again. You constantly need to test the right email subject line and content mix to connect with today’s busy subscriber.


Curiosity, when used correctly, is one of the most impressive aspects of email subject lines. It’s effective because it builds intrigue. It’s very important to remember that curiosity should not be used in a click-bait manner. You want to build anticipation for your emails by teasing a preview of what’s to come inside. Trickery is often employed as a curiosity element, but repeated trickery, especially with subscribers is a terrible practice. Even George Bush will only fall for a trick once. “Piqued my curiosity” is a common phrase that says it all. Curiosity is aroused when there’s an element of mystery involved. If you already know what’s in the email, there’s no chance for building curiosity.

A common misconception with curiosity in email subject lines is that your message always needs to be posed as a question. While asking a question is the most common way to employ curiosity in creative email subject lines, it’s not the only way. If you decide to use a curiosity question make sure it’s an open ended question where a simple “yes” or “no” response can’t work. For example:

BAD: Are you Ready for Winter?
ME: Yes. (Delete)

GOOD: Want to spoil yourself like an Oscar winner?
ME: Yes. (Open)


Read this paragraph before it’s too late!” Urgency is one of the oldest tricks in the marketing playbook. You can’t tell people to “Hurry!” all the time or they will tune you out. Urgency, when combined with relevancy and value, is one of the most compelling tools you can use in email subject lines. A retailer who constantly offers 25% off discounts can expect to see a rapid decline in interest and sales after the third or fourth time because people now expect a discount. They know that sale will still be there next week and the week after that. So why should they purchase anything right now? You need to give your subscribers a reason to do business with your company.

Some common ways to build urgency that still work well in email subject lines include the following:

  • Setting a deadline
  • Applying the principle of scarcity
  • Using active voice


Email subject lines are essentially useless if they’re not relevant. Your subscribers should be able to read your subject lines and instantly recognize their need to open that email. Creating relevancy in subject lines is easier said than done because certain elements are out of your control. People change over time which means the messaging you use needs to change too.

For example: A mother with a newborn baby might find an email about diapers, baby food, or formula very relevant to her current situation in life. Ten years down the road none of those subjects would be relevant to her anymore.
How do you harness relevancy? By connecting with the core values of your company and conveying those to your subscribers. What do you stand for? Why should subscribers interact with your company? What message does your business promote? The values of your products and services are highly relevant to the ideal subscriber. Abandoning those values undermines your relevance. A great question to ask yourself before you create a relevant subject line: “Why do our subscribers care about this?”.


Value is generally defined as relative worth, merit, or importance. Essentially, value is the ability of your products and services to impact and improve the lives of your leads, prospects, and customers. Think about white papers for example. We don’t read them for fun, we read them to gain knowledge. The value isn’t in acquiring the knowledge alone, it’s in what we believe that knowledge will add to our lives; becoming better at our job, getting a raise, impressing co-workers. When we wear a cool shirt we value how others think we look cool wearing the shirt, not the shirt itself. To have a great relationship with subscribers, you need to add value to their lives.


“SHOP NOW!”, screams the subject line. Why ALL CAPS with an exclamation point at the end? Whose emotion is being conveyed? Is it the retailer’s or the subscriber’s emotion? Emotion is the most subjective element of C.U.R.V.E. It’s all about being authentic with the message you’re trying to convey in your email subject lines. We think Seinfeld does a pretty great job of summing up the overuse of exclamation points in this clip:

Emotions such as excitement, frustration, happiness, and sadness shouldn’t be taken lightly. When you have exciting news, let people know. Forget about using the all CAPS or slapping that exclamation point onto the end, just be authentic. A new album from your favorite band is exciting. Paying $29.99 for the pre-order is not exciting. “Happy Birthday!” conveys emotion. “Spend $20 and Save 15% on your Special Day!” does not. Emotion is the element that ties all of the other pieces of the subject line together. Find a genuine connection between the content and the audience and you’ll be well on your way to a compelling email subject line.

Staying ahead of the C.U.R.V.E

The C.U.R.V.E. method has served us well. It’s wide open for your interpretation. There are a million different opinions on which elements should be represented in a subject line and most are wildly different from business to business. Some of our favorite subject lines were total flops and some of the ones we thought were duds became our biggest winners. As long as you keep testing subject line elements, you’ll be on the track to develop an email program that truly connects with your audience in a personal, authentic manner.

Need some inspiration? Contact us. 

About the Author(s)

Alex Williams

SVP, Practice Lead, Cloud Consulting. As one of Trendline’s first hires, Alex built out Trendline’s full service email practice. He is a true full stack email marketer – from strategy and creative to technology and analytics. Widely recognized as an email strategy and creative expert, as an SVP at Trendline, Alex brings his passion and creative problem solving skills to Trendline clients. Alex has had a hand in just about every account that’s ever been on Trendline’s roster. He is a frequent keynote speaker on topics related to design, technology, and their intersection in the inbox. Follow Alex on Twitter

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