There have been 12 different iterations of the iPhone over the eight years of its existence. While it might not have changed all that much from the outside, there has been a huge amount of upgrading going on inside.
From its dimensions to the screen size, pixel density, and battery life, we've charted some of the significant differences in the iPhone from the first model to the forthcoming iPhone 6s and iPhone 6s Plus.
While the phones have become taller and wider over time, they have also become thinner - with models after the iPhone 6 being the slimmest yet.
Surprisingly given its age, the original iPhone isn't the chunkiest - that honour goes to its successors, the iPhone 3G and 3GS.
As would be expected, in the eight years since the launch of original iPhone, the speed of the processor has increased considerably. The introduction of a dedicated co-processor from the iPhone 5s onwards helped free up even more processing power and extended the battery life.
Helping to drive this performance increase has been Moore's Law, which predicts a doubling of transistor density every two years - although that pace is starting to slow. As the manufacturing process for silicon chips evolves to etch out ever finer details - as small as 14 nanometers in the processor used by the iPhone 6s - more and more transistors can be crammed onto the chip. Electronics engineers use these additional transistors to create processors that are more powerful, faster, and more efficient.
Phone performance, measured here by Primate Labs' Geekbench 3 processor benchmark, has increased dramatically since the days of the iPhone 3GS. Single-core performance for iPhone 6 and 6 Plus is ten times that of the 3GS, while multi-core performance is nearly seven times that of the the iPhone 4s (the first iPhone with a dual-core CPU). Leaked Geekbench 3 numbers for the most recent 6s and 6s Plus models are around 2000 and 3500 points for single-core and multi-core performance respectively.
This chart shows a pretty steady expansion in the storage on the iPhone over time. That's just as well - as the iPhone camera has improved, so the size of the images taken has increased. And apps have got a lot, lot bigger too: earlier this year Apple increased the maximum of iOS apps to 4GB, which means just one app could fill the entire memory of the smallest original model.
With the launch of the iPhone 6s some have complained that Apple is still selling a 16GB model, but it's worth noting that when the iPhone launched, that was the largest storage tier sold.
The iPhone started at 3.5 inches and has developed through 4 inches and 4.7 inches to full 5.5-inch 'phablet' status. At the same time, the pixel density has gone from 163ppi to 401ppi. (Sony's latest Xperia z5 Premium has a startling pixel density of 806ppi).
This shows just how rapidly the pixel count has increased.
The screen is the main power draw in a smartphone: as iPhone displays have become physcially bigger and packed in more pixels per inch, so the battery capacity has had to keep pace.
3G talk time has increased by five times since it was introduced in the iPhone 3G, while video playback time (HD in later models) has doubled, and audio playback has more than trebled. But throughout all the iterations, wi-fi browsing time has only doubled - perhaps a reflection of how much more complicated and power-draining web pages have become in the last eight years.
The iPhone's camera has evolved from being a respectable snapper, capable of capturing a two-megapixel image, to having a 12-megapixel image sensor as well as autofocus, auto HDR, exposure control and many other features. Video quality has followed a similar path, from recording rather muddy 640 x 480 resolution footage to films that match the sharpness of the latest 4K televisions.
As is essential in this age of the selfies and video calling, the front-facing camera on the iPhone 6s and 6s Plus can capture five-megapixel stills and 720p video - a far cry from the rear camera-only early handsets.