What is this 2x2 part with one corner curved up?

Question

I recently came across this LEGO part:

What part is this? What can it be used for?

Answer 1

The part is Plate Special 2 x 2 with Double Inverted Curve, 1 Stud (4190).

It was introduced in a few sets in 2023. It's original use was to provide an upward curve on roofs. This can be seen very easily in Himeji Castle (21060):

It can of course be used for many other things. Trivially, these parts can be tiled to create small peaks in a 4x4 area:

These parts are also compatible with some of the other recent inverted curved slope elements. For example, here it is next to Brick Curved, Internal Double, 3 x 3 with 1/3 Inverted Cutout, Corner (73682):

This is the same profile as Brick Curved, 3 x 1 with 1/3 Inverted Cutout(70681):

This curve also aligns perfectly with the curve of the telephone bow, assuming that is can be inverted:

Here's one way those parts can be used together:

Because the telephone lines up with a pair of these, the 2x1 bow slope also lines up easily:

The notched variant (29119/29120) of those slopes are also compatible:

In fact, those parts can be used to fill both sides of this part at once:

There are plenty of ways to use this part, either by integrating with other slopes, or arranging it using SNOT techniques. For example, here's a quick window that I made to demonstrate how these can be combined to create a nearly circular outline: