Grid

The grid is the regular set of allowed points, like graph paper, that every corner of a layout shape is supposed to sit on.

Factories and mask writers do not accept arbitrary decimals. A process publishes a manufacturing grid, often 0.005 micrometer or 0.001 micrometer. The editor shows a coarser drawing grid so the crossings can be seen. On screen they are faint dots or lines behind the shapes. With snap turned on, a cursor jumps 0.05 micrometer at a time even when the file can store a step of 0.001. Corners that miss the manufacturing grid become off-grid errors, or a later tool rounds them without being asked.

The gold rectangle's corners sit on the dots. Those dots are the allowed points.

The finest step the file can store is a DBU. Snap is the habit of landing on the crossings. Some rules also want edges on a routing pitch that is many grid steps wide. A tidy on-screen grid is a drawing aid. The grid in the PDK is the one the foundry published, and that published grid is the one that wins.

The articles the grid, database units, and snapping and off-grid stay with those crossings.