Arrout is a commune in the Pyrenees mountains in the former province of Couserans some 12 km south-west of Saint-Girons and 3 km north of Castillon-en-Couserans. Access to the commune is by a country road from the end of the D404, which comes from Cescau in the south-east, to the village or by the Chemin d'Arrout from Audressein in the south. There is also a country road from Alas in the north-east. The commune is rugged and heavily forested throughout.
The Lez river forms the south-eastern border of the commune as it flows north-east to join the Salat at Saint-Girons. The Cayssau stream rises in the north-west and forms the north-western border before joining the Ruisseau de Lachein. The Ruisseau de Lasquert rises in the centre of the commune and flows north-east to join the Ruisseau de Lachein which flows south-west to join the Lez.
In 2010 the commune had 75 inhabitants. The evolution of the number of inhabitants is known from the population censuses conducted in the commune since 1793. From the 21st century, a census of communes with fewer than 10,000 inhabitants is held every five years, unlike larger towns that have a sample survey every year.[Note 1]
^At the beginning of the 21st century, the methods of identification have been modified by Law No. 2002-276 of 27 February 2002, the so-called "law of local democracy" and in particular Title V "census operations" allows, after a transitional period running from 2004 to 2008, the annual publication of the legal population of the different French administrative districts. For communes with a population greater than 10,000 inhabitants, a sample survey is conducted annually, the entire territory of these communes is taken into account at the end of the period of five years. The first "legal population" after 1999 under this new law came into force in 1 January 2009 and was based on the census of 2006.