The blue frame is 20x20 and positioned at y=160
The white frame is 20x20 and positioned at y=180
Obviously, these frames don’t appear to be overlapping. However, 160+20=180, so shouldn’t the bottom row of pixels on the blue frame be at the same location as the top row of pixels of the white frame?
To grab a quote from MSDN, “By convention, the right and bottom edges of the rectangle are normally considered exclusive. In other words, the pixel whose coordinates are ( right, bottom ) lies immediately outside of the rectangle.” - this is an ubiquitous convention.
In ROBLOX, 20 is not a center of a pixel - it’s a left-top corner of a pixel. So having the rectangle at 0 with width 20 means that it touches the left top corner of the pixel #20 (without overlap). Rectangle at 20 with width X similarly touches the left top corner of the pixel #20 - so two rectangles touch (but don’t overlap).
(even if it were in a center we’d use the top/left edge filling convention so right/bottom edges would be excluded; but in pixel-corner formulation it’s obvious that rectangles should not intersect)