Since we're talking about the grid system, I thought some people would be interested in this. I was intrigued about it since I first seen it. I was recreating it in Illustrator to try and understand the thought behind it. Here are a series steps of making the grid from what I seen.
The new shape is no longer a rounded rectangle, it's a superellipse. This shape can be made by starting with a circle and dragging the asymptotes to the edge of the shape's bounding box. I quite like the shape, with a rounded rectangle, the circles that make up the corner are very noticeable, this is a more subtle shape.
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The outer circle is positioned to match up with the mid-point of the corners, where the corners change directions. I guess you'd call it the inflection point.
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Then, the golden ratio is used to establish the vertical and horizontal lines here.
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The inner circle matches up with the inside of the square that's created by the vertical and horizontal lines, and the outer circle matches up with that same square but from the outside. And then the diagonal lines appear, although they could have been on the very first image.
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They might even conform to Apple's hardware, and considering that Jony is the designer of both, perhaps he is just adopting grid systems for software from ones he's been using for hardware.
Some of the icons don't seem to conform to the grid at all, like the camera icon maybe. I might superimpose the grid over all the icons to see if they somehow match up. Anyway, I like this sort of stuff.