Every berk's heard o' the rule of threes, right? Well, three ain't the only number that's got planar significance. Here's a list of numbers with some meaning attached to them. By the Guvners that is.

0: Zero, the big nothing. Any number of cultures claim to have invented it, and you might be forgiven for thinking that a number representing nothing would mean nothing. But in fact, zero is used to represent, amongst other things, the Doomguard ideal (zero activity), the Bleaker philosophy (zero meaning), and the Outlands, which are often used as an arbitrary zero of potential by planographers due to their neutral outlook.

1: The number one represents two of the three great rules of the multiverse. The unity of rings, because rings are singular and unitary, and the centre of the Multiverse rule, because it centers on the individual. Also, the plane of Mechanus, with its single layer, has an affinity for one, reflected in the singular nature of the modron mind. Speaking of which, both Primus and monodrones are associated with the number one. The Sign of One faction refers to its conception of God as 'the One', and the splinter group the Will of the One have taken the number as a symbol. Planologists associate the number one with the outlands, mainly because there are no zeroes on normal dice or playing cards.

2: The number two has been used in demonstrations of the polarity principle: ideas come in opposed pairs. Good and Evil, Law and Chaos, black and white, fiend and celestial, and so on. Many observe that by compromising, many polar pairs yield a third item, in accordance with the rule of threes. But on the other hand, many triads contain a pair of opposed concepts, thus directing us back to the polarity principle. The number two is also linked to Bytopia, the Twin Paradises, especially by Planologists, as is Arcadia, since the 'incident' described in Planes of Law.

3: The rule of threes is inescapable on the planes. Everywhere triads and triplets emerge. The factions are sometimes grouped in threes: the triad of order (Guvners, Mercykillers and Harmonium), the triad of despair (Dustmen, Doomguard and Bleak Cabal), the triad of the spirit (Sensates, Ciphers and Xaosmen), the triad of the self (Godsmen, Signers, Fated) and the triad of independence (Indeps, Anarchists and Athar) (see also five). Three is the number of the Beastlands, the Grey Waste, Arborea, Ysgard,and Arcadia. In planological dice-casting, three represents Baator, with its thrice three layers and three-armed eye symbol.

4: The rule of fours governs the Inner Planes. There are four elemental planes, four para-elemental planes, twice four quasi-elemental planes and half four energy planes. Four elemental lords rule the inner planes, and four genie rulers compete within them. Four is also the number associated with Acheron, Elysium, Gehenna and Pandemonium.

5: Five is the number of rules governing the powers, the number of the tenets of Xaos Theory, and the factions can be arranged in five threes (see above). They can also be grouped in three fives in a number of ways, such as: the pentad of law (Mercykillers, Harmonium, Guvners, Dustmen, Godsmen) who all believe in a grand scheme, the pentad of balance (Signers, Ciphers, Athar, Fated and Sensates) whose beliefs have no bearing on the overall status of the universe, although they all seek nevertheless for truth, and the pentad of chaos (Doomguard, Xaositects, Anarchists, Indeps and Bleakers) whose ideas are against an enduring system for things. The number five is associated also with genies, after the manner of the five genie races. Five is the planological number of Limbo, which was once thought to have five layers. There are also five castes of base modron, therefore associating five with Mechanus as well as Limbo.

6: Six is another number associated with the inner planes, after the six main inner planes, being the elemental and energy planes. Six is also the number associated with Carceri by planologists, and in dice-casting also Mount Celestia (six comprehensible layers) and the Abyss (six hundred plus layers).

7: Seven is a holy number of ancient significance, and is associated with the upper and lower planes as groups, and with Mount Celestia. Less pleasantly, it is also associated with the Lords of the Nine, of whom seven are known.

8: Eight is considered an inauspicious number on many worlds, and certain orders of wizards are forbidden to speak it. It is also associated with the quasi-elemental planes, and with the inner planes as a whole, being composed as they are of three rings of eight planes each. In planological tarot cards, it represents the Astral, for its four dimensions (according to some theories), each of which can be moved along either way, for a total of eight directions. Eight is also associated with the Dark Eight, which may be one reason for the taboos against it. The Mercykillers codified their philosophy into eight tenets.

9: Nine is associated with the unmortal races, and specifically with the group consisting of Archons, Guardinals, Eladrin, Modrons, Rilmani, Slaadi, Baatezu, Yugoloths and Tanar'ri. Nine is also the number of Baator in planological parlance, and of the Lords of the Nine.

10: Ten is another number associated with the Outlands, and with modrons, who have ten orders of hierarchs including Primus. Ten is also sacred to pure-blooded beholders, as it counts their eyestalks.

11: Eleven is another number associated with unmortals, this time adding Gehreleth and Aasimon to the list under 'nine'.

12: Twelve is the number of pseudo-elemental planes (para- and quasi-elemental), which together with the six main planes (see six) give the total of all the inner planes.

13: An unlucky number at the best of times, but especially when you consider that the Guvners catalogued the realm of Beshaba, maid of misfortune, in the Abyss, thirteenth. Thirteen is another number associated with the Abyss in planology. There are also thirteen true factols.

14: Fourteen is the number of factions according to the Free League, who try not to be one, but is otherwise insignificant.

15: Fifteen factions war for the minds and hearts of Sigil's citizens, and Cagers consider it to be a number of deep significance. There are also fifteen castes of modron.

16: Sixteen planes make up the Great Ring, and sixteen gate-towns stand in the Outlands. Also, there are sixteen material inner planes.

17: There are seventeen outer planes, for no good reason except that that's the way it turned out.

18: There are eighteen inner planes, expressed as either 2+4+4+8 (see four) or 6+12 (see six and twelve) or 2+16 (energy planes plus material planes).

19: If you include the semi-elemental plane of Shadow, there are 19 inner planes.

38: There are, according to normal cosmology, thirty-eight major planes of existence. However, see below.

39: If you believe the Ordial Plane theory (see 'mapping the infinite' on Jon's mimir), there are in fact thirty-nine major planes of existence, which would suit the numerologists a treat as it's three thirteens, and they'd be able to start invoking the rule of three all over again. Or you could draw on Shadow, if you preferred another way to make up the numbers.

666: The number of the beast. To snap back into the 'real' world for a second, they key passage in scripture is:

"Here is wisdom. Let him that hath understanding count the number of the beast: for it is the number of a man; and his number is Six hundred threescore and six"

- Revelation 13:18

In Roman numerals, 666 is all the numerals except M (1000) written in order: DCLXVI. Back in Planescape, the Guvners once thought that there were 666 layers in the Abyss. They were wrong. To date, 679 have been discovered.