Why do inserters insert into underground belt on the left?

[crackalackin]

I just noticed that inserters always insert on the right side of the belt. Except into a connected underground belt? Is this intentional? Is there a way to change that to stick to the right (without mods, I know Bob's can change the placement).

In the attached picture, the top and second work as expected. But as soon as the underground belt is connected, they both insert topside. It's the same if you rotate it 90 degrees. Is this a bug?

[crackalackin]

I have heard, it seems to be based on the direction of the belt and not the inserter. If that is the cause, that would explain this and I understood the inserter direction wrong

[Tertius]

It's always the right side of a belt, seen from the perspective of the belt - not the perspective of the inserter. Seems strange, but that's always been the case.

There were uncountable threads and discussions about inserting positions in the past years, you're not the first one to ask this. There is a technical explanation why it is correct to choose this side, however I confess I forgot what it was.

[crackalackin]

Thank you for confirming and explaining.

[astroshak]

It’s the far side of a belt moving left or right from the perspective of the inserter.

It’s the right side of the belt from the perspective of the belt when the belt is moving towards, or away from, the inserter.

Yes, it’s weird like that.