Duplo is Lego-compatible. A 2x4 Lego brick fits exactly onto 1x2 Duplo studs. A Duplo brick is 2x the height of a Lego brick, and a Duplo plate is 1x the height of a Lego brick.
It's fun to build large structures quickly with Duplo then detail them with Lego!
Oh cool, never knew about that one. I do have one more to add but it's mostly technics warmed over, which is 'Znap', clearly aimed at the same market as 'Knex', which Lego at some point saw as a competitor. Since then they've come their senses, the Znap line was ugly and highly impopular, leading to being discontinued after only a year.
What if I told you there is another plug type, that might still be sold with some old sets somewhere, and is still supported by a bunch of third party hubs (that for a long time were more advanced in many ways than Lego's own system ...)
Lego's stewardship of all this was a mess, and I'm super glad they open the system to officially blessed third parties.
There is 'duplo' and there is 'Modulex':
https://www.newelementary.com/2017/11/old-bricks-what-are-mo...