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They did not skip, per se. The very first iPhone used second generation telecommunication technology. Its second iteration used third generation tech, hence they decided to add 3G to the name (3G does not mean iPhone 3). iPhone 3Gs, the s version of iPhone 3G, was the third iteration. Starting from fourth iteration, they used iteration number in the nomenclature, hence calling it the iPhone 4.

right so accord. to the nomenclature, they "skipped" 2 plain & simple.
 
  1. iPhone 2G
  2. iPhone 3G
  3. iPhone 3GS
  4. iPhone 4
  5. iPhone 4s
  6. iPhone 5
  7. iPhone 5c
  8. iPhone 5s
  9. iPhone 6
  10. iPhone 6 Plus
  11. iPhone 6s
  12. iPhone 6s Plus
Technically the 10th iPhone would be either the 6 or the 6 plus. So get one of those, install iOS 10 on it when available and boom you'll have achieved you dream of running iOS 10 on the 10th iPhone.

The 6 and 6S plus are just a product variation of the 6 and 6S generation released at the same event, with nearly identical internals.

Its not like the Samsung S series and Samsung Note series which differ quite a bit.
 
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