:dart (c/3 ortogonally, p3) Found by David Bell, May 1992.

	.......O.......
	......O.O......
	.....O...O.....
	......OOO......
	...............
	....OO...OO....
	..O...O.O...O..
	.OO...O.O...OO.
	O.....O.O.....O
	.O.OO.O.O.OO.O.

:dead spark coil (p1) Compare spark coil.

	OO...OO
	O.O.O.O
	..O.O..
	O.O.O.O
	OO...OO

:density The density of a pattern is the limit of the proportion of live cells in a (2n+1)×(2n+1) square centred on a particular cell as n tends to infinity, when this limit exists. (Note that it does not make any difference what cell is chosen as the centre cell. Also note that if the pattern is finite then the density is zero.) There are other definitions of density, but this one will do here.

In 1994 Noam Elkies proved that the maximum density of a stable pattern is 1/2, which had been the conjectured value. See the paper listed in the bibliography. Marcus Moore provided a simpler proof in 1995, and in fact proves that a still life with an m × n bounding box has at most (mn+m+n)/2 cells.

But what is the maximum average density of an oscillating pattern? The answer is conjectured to be 1/2 again, but this remains unproved. The best upper bound so far obtained is 8/13 (Hartmut Holzwart, September 1992).

The maximum possible density for a phase of an oscillating pattern is also unknown. An example with a density of 3/4 is known (see agar), but densities arbitrarily close to 1 may perhaps be possible.

:D-heptomino = Herschel

:diamond = tub

:diamond ring (p3) Found by Dave Buckingham in 1972.

	......O......
	.....O.O.....
	....O.O.O....
	....O...O....
	..OO..O..OO..
	.O....O....O.
	O.O.OO.OO.O.O
	.O....O....O.
	..OO..O..OO..
	....O...O....
	....O.O.O....
	.....O.O.....
	......O......

:diehard Any pattern that vanishes, but only after a long time. The following example vanishes in 130 generations, which is probably the limit for patterns of 7 or fewer cells. Note that there is no limit for higher numbers of cells - e.g., for 8 cells we could have a glider heading towards an arbitrarily distant blinker.

	......O.
	OO......
	.O...OOO

:dinner table (p12) Found by Robert Wainwright in 1972.

	.O...........
	.OOO.......OO
	....O......O.
	...OO....O.O.
	.........OO..
	.............
	.....OOO.....
	.....OOO.....
	..OO.........
	.O.O....OO...
	.O......O....
	OO.......OOO.
	...........O.

:dirty Opposite of clean. A reaction which produces a large amount of complicated junk which is difficult to control or use is said to be dirty. Many basic puffer engines are dirty and need to be tamed by accompanying spaceships in order to produce clean output.

:diuresis (p90) Found by David Eppstein in October 1998. His original stabilization used pentadecathlons. The stabilization with complicated still lifes shown here (in two slightly different forms) was found by Dean Hickerson the following day. The name is due to Bill Gosper (see kidney).

	.....OO................OO....
	......O................O.....
	......O.O............O.O.....
	.......OO............OO......
	.............................
	....OO..................OO...
	....O.O..........OO....O.O...
	.....O..........O.O.....O....
	..O.............OO.........O.
	..OOOOOO........O.....OOOOOO.
	.......O..............O......
	....OO..................OO...
	....O....................O...
	.....O..................O....
	..OOO..O..............O..OOO.
	..O..OOO........O.....OOO...O
	...O............OO.......OOO.
	....OO..........O.O.....O....
	......O..........OO....O..OO.
	....OO..................OO.O.
	.O..O....................O...
	O.O.O..OO............OO..O...
	.O..O.O.O............O.O.OO..
	....O.O................O..O..
	.....OO................OO....

:dock The following induction coil.

	.OOOO.
	O....O
	OO..OO

:domino The 2-cell polyomino. A number of objects, such as the HWSS and pentadecathlon, produce domino sparks.

:double-barrelled Of a gun, emitting two streams of spaceships (or rakes). The following diagram shows a double-barrelled p104 glider gun. This gun was found by Noam Elkies in March 1996 (except that Elkies used blockers instead of molds, the improvement being found by David Bell later the same month).

	.OO....................................
	.OO.................O..................
	...................O.O............O.O..
	....................O............O.....
	OO.......OO.......................O..O.
	OO.O.....OO.......................O.O.O
	...O.......................O.......O..O
	...O.......................OO.......OO.
	O..O.................OO.....O..........
	.OO..................O.................
	.....................OOO...............
	....................................OO.
	....................................OO.
	.OO....................................
	O..O...................................
	O.O.O................O.O....OO.....OO..
	.O..O.................OO....OO.....OO.O
	.....O............O...O...............O
	..O.O............O.O..................O
	..................O................O..O
	....................................OO.

:double block reaction A certain reaction that can be used to stabilize the twin bees shuttle (qv). This was discovered by David Bell in October 1996.

The same reaction sometimes works in other situations, as shown in the following diagram where a pair of blocks eats an R-pentomino and a LWSS. (The LWSS version was known at least as early 1994, when Paul Callahan saw it form spontaneously as a result of firing a LWSS stream at some random junk.)

	.OOOO.....OO....
	O...O......OO.OO
	....O......O..OO
	O..O............
	................
	.............OO.
	.............OO.

:double caterer (p3) Found by Dean Hickerson, October 1989. Compare caterer and triple caterer.

	.....OO...O........
	....O..O..OOO......
	....OO.O.....O.....
	......O.OOOO.O.....
	..OOO.O.O...O.OO...
	.O..O..O...O..O.O..
	O.O..O...O.OO....O.
	.O..........OO.OOO.
	..OO.OO.OO...O.....
	...O...O.....O.OOO.
	...O...O......OO..O
	.................OO

:double ewe (p3) Found by Robert Wainwright before September 1971.

	......OO............
	.......O............
	......O.............
	......OO............
	.........OO.........
	......OOO.O.........
	O.OO.O..............
	OO.O.O..............
	.....O...O..........
	....O...OO....OO....
	....OO....OO...O....
	..........O...O.....
	..............O.O.OO
	..............O.OO.O
	.........O.OOO......
	.........OO.........
	............OO......
	.............O......
	............O.......
	............OO......

:double wing = moose antlers

:dove The following induction coil.

	.OO..
	O..O.
	.O..O
	..OOO

:down boat with tail = cis-boat with tail

:dragon (c/6 orthogonally, p6) This spaceship, discovered by Paul Tooke in April 2000, was the first known c/6 spaceship. All other known c/6 spaceships are flotillas involving at least two dragons.

	.............O..OO......O..OOO
	.....O...OOOO.OOOOOO....O..OOO
	.OOOOO....O....O....OOO.......
	O......OO.O......OO.OOO..O.OOO
	.OOOOO.OOO........OOOO...O.OOO
	.....O..O..............O......
	........OO..........OO.OO.....
	........OO..........OO.OO.....
	.....O..O..............O......
	.OOOOO.OOO........OOOO...O.OOO
	O......OO.O......OO.OOO..O.OOO
	.OOOOO....O....O....OOO.......
	.....O...OOOO.OOOOOO....O..OOO
	.............O..OO......O..OOO

:drain trap = paperclip

:dual 1-2-3-4 = Achim's p4