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After your sender name, your subject line has the biggest impact on whether your email is opened or not. Its critical importance is why it’s the most tested email element and why it’s vital to stay on top of the latest subject line trends.

Driven by mobile and social’s influence on email marketing, here are four key trends to keep in mind as you craft your subject lines this year:

1. Mobile continues to shorten subject lines.
Because mobile devices display fewer characters of a subject line than their webmail and desktop email client counterparts, the growth of mobile email reading is putting downward pressure on subject line lengths.

Thirty-one percent of marketers say at least half of their emails are read on mobile devices, according to our 2015 State of Marketing report. And, somewhat shockingly, another 23% of marketers are unsure how often their emails are viewed on mobile. Taking those two together, it’s likely that around 38% of brands have their emails read on mobile devices at least half of the time.

Because mobile email apps like the iPhone’s generally display around 35 characters of a subject line in the inbox view, marketers are increasingly writing subject lines that land in the 20 to 40 character range, which tend to maximize their effectiveness. It’s not that longer subject lines can’t be effective, but they just aren’t seen in their entirety until the email is opened, when their effect is lessened.

2. Snippet text becomes increasingly important “second subject line.”
Counter-balancing shrinking subject lines is the fact that more subscribers are able to see snippet text, which gives subscribers a preview of the content of the email and appears next to or underneath the subject line in the inbox view of the native iPhone email app, Gmail, Yahoo Mail, and other email clients. For instance, the native iPhone email app displays around 80 characters of snippet text, more than twice the number of characters that it displays of a subject line.

Because it has the ability to extend and support the subject line, snippet text is an important third element of email envelope content. However, Salesforce Marketing Cloud research last October found that only 47% of B2C brands were fully optimizing their snippet text. The majority of marketers have snippet text that consists of nonsensical URLs or static administrative email copy, missing out on the opportunity to have a strong “second subject line.”

3. More images appear in subject lines.
Subject lines aren’t limited to text anymore. Starting in 2012, there was good support for special characters like hearts, stars, and arrows. And now the native iPhone email app, Outlook.com, and other email clients support emojis. Currently around 2% of B2C subject lines include special characters or emojis.

Using special characters and emojis in subject lines may seem gimmicky or silly—and many implementations are very simplistic. However, adding these visual elements to subject lines have proven quite effective for some brands—which should come as no surprise since adding images to tweets, blog posts, any just about any other media also improves response.

Also, some uses of special characters and emojis are truly sophisticated, as you can see in Salesforce’s 100 Inspiring Subject Lines collection. For instance, in a Sept. 5, 2013 subject line, Jeep uses bullet and vertical bar characters to create an impressive facsimile of their logo.

In an Aug. 20, 2014 subject line, the Democratic National Committee combined three emojis, two symbols, and two abbreviations to create a kind of pictogram subject line.

And in a Nov. 26, 2014 subject line, Moosejaw creates a $100 bill using special characters. That’s some impressive craftsmanship.

If this trend continues and inbox providers eventually add subject line support for custom images, including logos, then I wonder if we’ll see the rise subject line designers.

4. More hashtags appear in subject lines.
In the age of omni-channel campaigns, hashtags in subject lines connect email campaigns with social media campaigns. Often, using a hashtag in a subject line sacrifices a small percentage of opens in exchange for driving more social activity, so they should be used thoughtfully — and they tend to be. Currently, well under 1% of B2C subject lines include hashtags.

Hashtags only first started appearing in 2013, so this is a young trend with room to grow. While hashtags are great for tapping into larger narratives and themes like #GivingTuesday, more brands are creating hashtags for their own events. Plus, few brands currently use hashtags in subject lines as rhetorical devices for humor. That will likely change during 2015.

While each of these trends stands on its own, they are also at least partially related. Mobile shrinking subject lines is in many ways the driving force behind the other three trends, which are all tactics for compensating for shorter subject lines. Optimizing your snippet text gains you more impactful envelope content; using special characters and emojis let you communicate visually in subject lines; and hashtags let you tap into big ideas and themes.

As you craft your subject lines for your upcoming campaigns, keep these trends in mind and do some A/B testing to see which tactics your subscribers respond to best.