Seam bowling is generally classed as a subtype of fast bowling, although the bowling speeds at which seam can be a factor include medium-pace bowling. Although there are specialist seamers that make deliberate use of off-cutters and leg-cutters at the expense of bowling slower than regular fast bowlers, most bowlers employ the seam to some effect so therefore the terms "seamer" and "fast bowler" are largely synonymous.
In order to achieve this effect, a seam bowler usually delivers the ball with the seam held upright, with rotation about a horizontal axis. This keeps the seam aligned vertically as it travels towards the batsman, making it likely that the ball will bounce with the seam on the pitch.
The direction and degree of deviation from a straight path are dependent on the small-scale alignment of the seam and any irregularities in the pitch surface. This means that deviation caused by seam is chaotic and unpredictable.
However, it is also possible, by holding the seam at an angle and rolling the fingers over the surface of the ball, to produce a deliberate off cutter in which the ball veers away from the off side when it bounces on the pitch, or leg cutter in which it veers away from a right-handed batsman. Former Australian bowler Dennis Lillee employed a leg cutter of this sort to considerable effect; however, deliveries of this kind will be slower than if the bowler simply bowls with the seam upright, hoping for movement one way or the other. Some bowlers deliberately use cutters more for their surprise slowness than the deviation off the pitch - a tactic that can also be effective.
Often the deviation caused by seam is not large enough to cause a batsman significant problems with playing the ball. Occasionally, however, the ball can deviate far enough to hit the edge of the cricket bat instead of the middle, producing a catch for nearby fielders. Swing bowling is a way of getting greater deviation, but is harder to control.
Australian fast-medium bowler Glenn McGrath has used his seaming ability to great effect in his distinguished career. The ball 'seams' at its best at the start of a teams innings, when the ball is new. A pitch which has cracks in it may assist a seam bowler as well.