It is said that the gods play games with the lives of men. This fact is more apparent on the Discworld than on many other worlds - the Gods of the Disc being of the noisy and attention seeking kind.
It is also said that everything that could happen does happen somewhere. This theory is referred to as the Trousers of Time, and which leg you happen to go down depends on whether you turn right or left at every moment of your life. Attempts to explain this theory in greater detail involve exceedingly long words, headaches for everyone involved - particularly the person doing the explaining - and tend to end in the vastly simplified form: because of quantum.
Certain things were true in several legs of the Disc's trousers:
a pair of dwarves, a married couple, went walking through the woods near the entrance to their mine. There, in almost every universe, they discover four things:
1) several burning carts
2) accompanying dead bodies
3) an entirely unmagical sword
4) the sole survivor - a human baby with a crown shaped birthmark.
It is a well known fact that people find the young of other species almost unbearably cute. There may be some exceptions to this rule, such as baby slugs or spiders, but it is generally the case; even in people who have no fondness for the young of their own species, or who lack any desire to procreate themselves. Perhaps this explains why, in most universes, the dwarf couple take the child in, shelter it through the long winter, and become attached to it. So much so that they end up raising the child as their own, in accordance with dwarf tradition. They are so successful that the child, despite exceeding six feet in height, does not suspect that they are not, in fact, a dwarf after all.
The possible fates of this human/dwarf child are endless and all, in some sense, exist somewhere. Sometimes the child dies young, from illness or in a mining accident. Sometimes the child spends their entire life down a mine, among dwarves, and is happy.
In most cases however, the child eventually leaves the mine. The Discworld runs largely on narativium, and a child found alone in the woods with a sword and a suggestive birthmark is a child for which the gods have plans.
At this point we need no longer consider all the possible trouser legs and can instead focus on just two. They are, after all, remarkably similar: in both, the child (known as Carrot) is being sent to Ankh-Morpork to join the much maligned City Watch; in both, unscrupulous men plot to bring dragons back into the world, fear into the hearts of the city's residents, and power into their own hands; in both the gods watch and role dice and play by rules no one else knows.
In fact, the two are identical save for one small detail. One event, one instant in the shared past of these two worlds went slightly differently. So that, although the two youngsters travelling the long road from the city may look alike in every detail, there is one important difference.
One chromosome's worth, to be exact.
Neither version of Carrot has any idea of what lies ahead of him in Ankh-Morpork, but one of them will at least never have to worry about being crowned it's king.
People who have no idea that Carrot is of the female persuasion, a partial list:
- Commander Vimes - to be fair to him, he was massively hungover when they first met. Also, completely boggled, both that somebody had volunteered to join the Watch, and that the person in question was six-foot tall and claiming to be a dwarf. Noticing anything else odd was, at the time, entirely beyond him in the face of those two impossibilities. Afterwards, there was a box marked 'Carrot' in Vimes' head. The list of associated attributes included items like 'tall' and 'painfully earnest' and 'king?!?' and, yes, 'male' but Vimes didn't think about that one any more than he did the others; which was never at all, if he could possibly help it.
- Sergeant Colon - it would never occur to him that a woman could do his job. Well, unless she was a werewolf (Angua) or a vampire (Sally) or a dwarf (Cheery).
- Corporal Nobbs - although since Klatch he has occasionally wondered...
- Sergeant Detritus - he hasn't quite grasped the actual difference between men and women in any case. Humans are just so squishy and confusing.
- Sergeant Cheery Littlebottom - Captain Carrot is a dwarf and all dwarves are male. She may be progressive personally, but social conditioning is difficult to overcome. We can rarely see our own blind spots. Speaking of which...
Some people who know perfectly well:
- The Ladies of the Seamstresses Guild - they've all been there and know the signs. Plus, a teenage boy that uninterested in their collective charms? Please. They think Carrot is following the fine tradition of girls disguised as boys, they haven't realised that Carrot doesn't know, or they would have taken her aside and explained a few things a long time ago.
- Vetinari - he makes it his business to know everything, especially when it concerns people with mysterious, possibly royal, pasts turning up in his city. Since a long lost queen is likely to be far less of a threat than a king, he's happy to leave well enough alone. He's vaguely looking forward to the day Vimes either figures it out or (far more likely) has it pointed out to him. The man's face will be a picture.
- Lady Sybil - thought for a long time that it was one of those things that everyone knew and just didn't talk about. Then realised that, no, her husband actually is that stupid.
- Angua - the nose never lies. Now she just needs to decide what to do with the information.
Between Angua’s choices and Carrot’s possible responses, it seems some quantum tailor is likely to be kept very busy indeed.
 These trousers are, of course, strictly metaphorical. Thereby accounting for the fact that a disc could posses trousers at all, let alone a pair with (far) more than two legs.
 But not all of them. All possibilities must exist somewhere, even the unpleasant ones.
 Literally, Vetinari intends to have an iconographer standing by.