Standard-cell power rail (followpin)
Beginner
A thin power wire that runs along each row of small parts and touches every one, like a local street.
Novice
Thin supply and ground wires on the lowest wiring layer, running along the top and bottom edge of every row of logic cells. Cells are designed so their power connections touch these rails when they are placed side by side in a row.
Expert
Called followpin rails because they follow the cell rows. They are the most resistive part of the grid, so stacks of vias from the straps above land on them at regular intervals. The rails take room inside every cell, which limits how short cells can be made; that is one motivation for buried and backside power.
Explained in Power planning (Design Flow).
See also: Power strap (stripe), Standard cell.