At least in KiCad, the footprints of through-hole pin headers have one squared annular “ring”, the other ones are round:

enter image description here

What does the square indicate?

(Also I’m wondering if there is perhaps a convention of connecting GND to it.)

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    \$\begingroup\$ have you also noticed that the square usually has a 1 label? \$\endgroup\$ – jsotola Mar 24 '19 at 16:47
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    \$\begingroup\$ I'd say clear duplicate. \$\endgroup\$ – Marcus Müller Mar 24 '19 at 20:37
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    \$\begingroup\$ At least for KiCad the answer is very simple. It is specified that way in the library convention: kicad-pcb.org/libraries/klc/F7.3 (Answer in comments as i can not create an answer to a duplicate question.) We are however slowly transitioning to using rounded rectangle pads instead of square ones. (Benefits: Better solder joint, more space for traces.) \$\endgroup\$ – Rene Pöschl Mar 25 '19 at 17:35

It’s just a quick and easy way to distinguish which connector pad is pin 1.

Particularly when there is no silkscreen present or just when routing the PCB.

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    \$\begingroup\$ And there is not a convention of connecting ground to it! Don't even think that thought! \$\endgroup\$ – TimWescott Mar 24 '19 at 19:27
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    \$\begingroup\$ Of course there's a convention of connecting ground to it. It's just that there are also 17 other conventions. \$\endgroup\$ – Russell Borogove Mar 24 '19 at 22:33

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