The "S" line of iPhones, which emerge every other year with minimal physical changes, are supposed to be the "boring" ones.
But take a closer look at the family, which range from the iPhone 3GS in 2009 to this year's iPhone 6S, which goes on sale on Friday, and you'll find Apple has introduced a pattern of subtle innovations that have helped shape the evolution of its blockbuster smartphone. They go against the grain of the wider belief that S iPhones are uninspired because they come in essentially the same chassis as their previous non-S iterations.
Features such as Siri, introduced with the iPhone 4S in 2011, or the Touch ID fingerprint sensor, which debuted as part of the iPhone 5S in 2013, seemed like gimmicks at the start. But both ended up igniting a trend in the smartphone industry, with Siri inspiring a crop of virtual assistants such as Google Now and Microsoft's Cortana, and fingerprint sensors now the norm among high-end mobile devices.
Touch ID, when paired with the Passbook digital wallet introduced in the iOS 6 version of its mobile software in 2012, became the building blocks for its Apple Pay mobile payment system, which emerged with the iPhone 6 last year.
Those innovations have kept interest alive with a phone that looks identical to the previous year's model. Keeping up the iPhone hype is critical to Apple, which generates more than two-thirds of its revenue from its smartphone business, and continues to dominate the industry at the high end.
Here's a deeper examination of what each "S" update has brought to the iPhone family.
3GS: S is for Speed
"The S simply stands for 'Speed' because this is the fastest, most powerful iPhone we've ever made."
That was the first (and last) time Apple shared its thinking behind the now-mysterious "S" that graces new iPhone model numbers in odd years. Presenting on stage at the company's developers conference in 2009, Apple's senior vice president of marketing Phil Schiller made it clear that although the iPhone 3GS looked identical to its predecessor, its innards made it twice as fast (on average) as the previous year's iPhone 3G.
Reviews for the 3GS were mixed. Yes, it was faster. The addition of video recording and voice control were nice features. But Apple was just playing catch-up. Competing phones from Nokia and others (including those powered by Google's Android mobile operating system software) already had these features (and more).
And that's largely how the "S" earned its reputation. Paltry updates mixed with technical issues later on (like overheating), left "S" owners less than proud of their incrementally better iPhones. The "S" was a half-step.
What wasn't obvious at the time was that Apple was laying the groundwork for what would later become one of the iPhone's flagship features: Siri. Schiller also introduced a new feature called "Voice Control" and described it as a simple way to call friends and family, or queue up a song. And that's pretty much all it did.
I asked around, and it seems that many people don't even remember Voice Control. It was that thing you accidentally activated once in awhile when you held the home button down for too long. (If that sounds familiar, it is because Siri seems to have inherited this annoying trait.)
Only two years later -- in another "S" phone -- did Voice Control re-emerge as Siri, the smart and sassy voice-activated virtual assistant.
4S: S is for Siri
Optimistic that the 3GS was a temporary design, most of us expected a fully-revamped iPhone 5 to succeed the iPhone 4. Instead, the iPhone 4S came along, and though Apple didn't utter the words, "S" obviously stood for Siri.
Well, it might have stood for other things, too: storage (up to 64 gigabytes from a prior high of 32 GB), speed (the A5 chip, designed by Apple, was its first dual-core processor), and "Seriously, you shot that on your phone?" The 8-megapixel rear camera quality was impressive.
At the time, the introduction of Siri on the 4S didn't seem like a big deal. But today, the virtual assistant has become a voice-activated control center that lets you access devices beyond the iPhone. Want to turn on your smart light bulbs? Siri can help. Can't figure out who's in that movie playing on your fourth-gen Apple TV? Siri's got the answer.
Storage, speed and Siri made the iPhone 4S a great upgrade for those still using the 3GS. But those on the iPhone 4 balked. Without the foresight of Siri's greatness, it was yet another "incremental upgrade."
5S: S is for Security
Unlike Siri, it was easy to see how Touch ID in the iPhone 5S would evolve into a killer feature. CNET editor Scott Stein called out the fingerprint security sensor as innovative. but noted it it had even more potential than saving you the trouble of typing in a password to access your phone:
"(It could act) as a password replacement for third-party apps or even a way to make payments, or check in to flights," Stein said. "It could be a mobile wallet killer app, and a companion to Apple's somewhat dormant Passbook app that launched with iOS 6. But those extra features won't be coming anytime soon."
Today, nearly all those things are true. The fingerprint scanner is the cornerstone of Apple Wallet, helping to make the mobile payment system a success where others (like Google Wallet) previously failed.
Touch ID is one of Apple's greatest iPhone innovations, but when it launched with the iPhone 5S, it just wasn't worth it. Upgrade to unlock my phone faster? No thank you, users said.
It's also worth mentioning that the iPhone 5S introduced Apple's M7 motion processor. Apple marketed it as tool for collecting data from the iPhone's internal accelerometer, gyroscope and compass and using it to provide you with fitness-tracking information since iPhone could now count your steps, But it was much more than that.
Since the M7 processor knows when you're moving, it can do things like cut back on network pinging while you're sleeping (not moving for long periods of time) or stop asking you to join Wi-Fi networks while you're driving.
iPhone 6S: S is for 'Say goodbye to the home button?'
It's too early to read the iPhone 6S's blueprint for the future, but we can speculate that 3D Touch, a feature that responds to hard-presses on the screen, will play a key role in shaping future iPhones.
At first glance, 3D Touch seems gimmicky. Pushing in and doing what feels like an Android "long press" could read like an incremental upgrade, but the potential is greater. For instance, future models could have a variety of pressure sensitivities. A light press would bring up a menu of options (as it does today), but a harder press could do something else -- like take you back to the home screen.
CNET's Roger Cheng and Scott Stein. The next iteration of the iPhone -- a non-"S" version due next year if Apple continues on its yearly update pattern -- could serve as the catalyst for big changes in the iPhone 7 and beyond.