Arbre is the planet on which Anathem takes place. It has an oxygenate atmosphere, relatively temperate climate, and is inhabited by more than one billion persons. It is ruled politically by the transient Saecular Power, and intellectually by the ascetic maths. Known languages include Fluccish, the common tongue of non-mathic peoples, and Orth, the language of the avout of the maths.

The written history of Arbre extends more than 5000 years into the past, albeit with a number of gaps and enigmas due to planet-wide political upheavals and disasters.

The word arbre means 'tree' in some Romance languages such as French and Catalan.

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