Finder pattern
The three large nested squares that tell a scanner where the code is.
The three large concentric squares in the top-left, top-right and bottom-left corners are finder patterns. Each is 7×7 modules: a solid ring, a one-module light gap, and a 3×3 dark core.
How a scanner uses them. Scanning any line through a finder pattern produces a dark:light:dark:light:dark run in the ratio 1:1:3:1:1. That ratio is rotation-invariant, so a decoder can find the patterns from any angle. Three of them give the code's position, size and orientation; the missing fourth corner is what tells the scanner which way up it is.
Why they must stay square. Styled QR generators often round the finder patterns into circles or blobs. Mild rounding usually survives because decoders are tolerant, but the further the 1:1:3:1:1 ratio is distorted, the more scans fail — particularly at an angle or in poor light. The proportions are not decoration.
In practice
Hold a QR code at 45 degrees and it still scans. That is the finder patterns working: the 1:1:3:1:1 run holds its ratio under rotation and moderate perspective, so the decoder can locate all three corners and compute the transform back to a square grid.
Now consider a styled code where the finder patterns have been drawn as circles. Flat-on it reads fine. At an angle, in poor light, on a slightly creased menu, the ratio has drifted far enough that detection fails — and the failure looks random to the person holding the phone, which is the worst kind of bug to diagnose.
Questions
Why are there only three, not four?
Three points define orientation. A fourth would be redundant, and the empty corner is itself the signal that tells the decoder which way up the code is.
Can I put my logo in a finder pattern?
No. Error correction covers data modules; finder patterns are structural. Replacing one usually stops the code being detected at all. Centre logos work because the middle of a code is data.