End rule is a type of stress rule, first introduced by Prince (1983)), which captures the delimitative and culminative properties of stress in grid theory. End Rules (ER) apply at the highest levels of the grid and assign an extra gridmark to (typically) the rightmost or leftmost prominent element.
In English, the End Rule that assigns main word stress applies domain-finally at the line indicated with number 3 and adds a grid mark to the rightmost column above the penultimate syllable:
3 * 2 * * * * 1 * * * * * * * * achromatic achromatic
- Prince, A. 1983. Relating to the Grid, Linguistic Inquiry 14, pp.19-100