During its Build 2015 developers conference in San Francisco, Microsoft revealed what happened to Windows 9, but it's taken until now for most of us to realise. Also, you might not be that satisfied with the answer.
When the company first announced Windows 10, many wondered why it was jumping from 8 and 8.1 straight to 10. It seemed odd considering Microsoft had adhered to a traditionally numeric theme in the years immediately prior.
Now we know though. It's because Windows 7 8 9 (seven ate nine).
Yeah, it's that old joke.
The revelation might be a bit lame but the actual way it was presented during Build was actually quite cool - especially, if you're a nerd like us. It was written in binary code on the front of Joe Belfiore's T-shirt as he presented new Windows 10 features on stage during the first keynote.
In fact, each of the four segments in the Windows logo on the shirt had a different message, and eagle-eyed developer Kévin Gosse worked out what they said after analysing an image of Belfiore.
As well as the joke, "Windows 10, because 7 8 9", the shirt said, "There are 10 types of people in the world." This refers to an old programmer joke, 10 is binary for two.
It also said, "Congrats on being one of the first." And, "Windows Insiders help us develop the future. Talk to us @Windows."
We're not sure what impresses us most, the T-shirt itself or Gosse's translating abilities. It's not the joke though. Oh no.