These days, I think however the most common reasons are:
- Aesthetics, to keep all spacing between key groups 0.25u
- Because Macintosh has F1–F12 and then an additional key, traditionally for Eject but more lately Lock Screen / Touch ID and many mainstream keyboard brands follow.

Unfortunately this will be cancelled, thank you to those who participated!

Heads up, this is only the second time I've written a post in English, so apologies in advance for any awkward phrasing. (*한국어 원문: https://brunch.co.kr/@ruseupi/251)

The first CNC-machined aluminum TKL cases were built by cannibalizing Cherry MX3000 boards for parts.
Back then, custom controllers (like LIMKB or ICON, Korean DIY controller projects) didn't exist yet, so the group-buy PCB and case relied entirely on donor parts from the MX3000, including its controller, switches, keycaps, and even stabilizers.




The black 3000 shown uses thick POM keycaps, while the light gray one uses thin PBT keycaps. However, the black MX3000 used to be sold with a Winkey layout and double-shot keycaps.




At the time, TKL layouts needed a separate key just to control Num Lock. Originally that spot held a Num Lock keycap, but today's boards usually place an F13 keycap there instead.




For reference, the Cherry MX1800 layout has an LED indicator to the right of F12. This is likely why the modern F13 position ended up there.




Of course, older keyboards had function keys all the way up to F24, but this post is specifically about why F13 shows up on TKL keyboards, so hope that clears up any confusion.
I originally posted this on r/MechanicalKeyboards, but wanted to share it here on Geekhack as well since I know a lot of you appreciate the history side of things.

In all my years I never made the mistake until last night, of getting habanero on my penis.
I washed my hands TWICE with different soap before handling, and it was clearly not enough.
I was in fairly intense pain for about 2 hours. Next time, gloves.