Comment 12 for bug 1738283

FWIW, I think this keycode represents the key between the left shift key and the first letter key on the fourth row, if there is one. European keyboards have one, and on e.g. a UK keyboard it types a \ unshifted and a | shifted - this is exactly how it looks in the en-gb keymap file:

# evdev 86 (0x56), QKeyCode "less", number 0x56
backslash 0x56
bar 0x56 shift
bar 0x56 altgr
brokenbar 0x56 shift altgr

The definition that somehow gets into the en-us keymap file appears to be actually how the key is intended to work on *German* keyboards:

https://en.wikipedia.org/wiki/German_keyboard_layout

Note how the key is labelled with <, > and | characters there. The French layout has the same key labelled with < and > but not |. So basically it seems like that same definition for this key shows up when you ask xkb for an en_US map.

Bonus historical note: modern US keyboards don't have a key there at all, they're 101/104-key keyboards, where the left shift key is very wide and the key next to it is the first letter key. But *old* US keyboards, specifically the 83-key 'XT' layout, *DID* have a key there!

https://en.wikipedia.org/wiki/IBM_PC_keyboard#/media/File:IBM_Model_F_XT.png

From that picture, the key was labelled with \ and | characters, like a modern UK keyboard (presumably this is where the modern UK keyboard derived its use for the key from). I wonder if there's a keyboard nerd out there somewhere with a working US XT keyboard who we could ask to press that key and see what keycode it generates...:) I suppose if it's this keycode, we could arguably report a bug in xkb that for en_US, that keycode should work like a modern UK keyboard (backslash / bar / bar / brokenbar), not a modern German keyboard...:)