The direction the grass blades lie on a putting green, which nudges the ball as it rolls. Putt with the grain and the ball runs faster and farther; putt into it and the green plays slow and grabby; across a slope, grain can add to or fight the break. A shiny, silvery look down your line means you are going with the grain — dull and dark means against. Bermudagrass grains hard and is the classic culprit; Poa and bentgrass grow upright and show little grain at all. Ignore the folklore about grain chasing the sunset or the nearest water — even the USGA says nobody knows why it sets, and slope still moves the ball far more.