Leading edge

Leading edge is the configuration of the front row of an active region. Usually, it is described as the number of cells, such as a two-cell leading edge, but nonconsecutive cells may require a more detailed description such as obo.

Moving objects

The leading edge of a moving object is the edge of that object's bounding box that is farthest in the direction of travel, essentially, the "front" of the object. The leading edge of a diagonally moving object can be considered in two ways: it may incorporate both edges of the bounding box in its direction of travel, or, alternatively, it may be defined as the diagonal row of cells that is farthest in the direction of travel. Most known moving objects have a leading edge that only ever moves forward; in 1991, however, Dean Hickerson constructed the 13-engine Cordership, the first non-monotonic spaceship -- a spaceship whose leading edge "falls back" in at least one generation.. In 1992, Hartmut Holzwart found the first orthogonal example. Since then many other spaceships have been found which exhibit this property, such as the weekender.

The trailing edge of a moving object may be considered the "back" of the object. It is the edge of the bounding box which is farthest opposite the direction of travel. For puffers and rakes this does not generally include their output. The trailing edge is described most commonly in reference to spaceships, as their shape often determines their reactions with tagalongs or other objects.