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"Remember Koom Valley!" Hundreds of years ago, the trolls and the dwarfs met in a battle at Koom Valley. It was a remarkable battle in that each side ambushed the other and, of course, in being the historical event symbolizing the enmity between trolls and dwarfs. Nowadays, dwarfs celebrate the Battle of Koom Valley Day, on the same day that trolls celebrate Troll New Year. Sometimes, in a city where both dwarfs and trolls live (e.g. Ankh-Morpork), the two groups are careful enough to plan their celebratory marches so that they are coincidentally on the same street (see Men at Arms).

Some notably intelligent people (e.g. Susan Sto Helit), after carefully studying history books, point out that the Battle of Koom Valley seemed to have been repeated several times. Either the trolls and dwarfs really do hate each other very much (which is, of course, the case), or somebody has been messing with the space-time continuum (see Thief of Time).