(Reuters) - Here is a look at the haj, the largest mass gathering in the world, which starts in Saudi Arabia on Wednesday:
* The haj is an annual pilgrimage that Muslims make to Mecca with the intention of performing religious rites as taught by the Prophet Mohammad to his followers 14 centuries ago. The haj is mandatory once in a lifetime for every able-bodied Muslim who can afford it and is the fifth pillar of Islam and the most significant manifestation of Islamic faith and unity.
* The annual haj begins in the 12th month of the Islamic year (which is lunar, not solar, meaning the haj and the fasting month of Ramadan can fall at different times in different years)
* Some 1.7 million foreign pilgrims have so far arrived in Saudi Arabia. Most pilgrims are from Asia, especially Indonesia which has the highest haj quota. The quota system was introduced in 1987 following an agreement by all member countries of the Organisation of Islamic Cooperation to limit the number allowed by each country to 0.1 percent of its population.
* Saudi Arabia said last week that it would not prevent Syria sending pilgrims, despite the civil war where more than 30,000 people have been killed since protests against President Bashar al-Assad erupted in March 2011.
* Stampedes, tent fires and other accidents have caused hundreds of deaths in the past 30 years, forcing the government to build new infrastructure. In 2011, Saudi Arabia began the biggest expansion yet of the Grand Mosque, to increase its capacity to 2 million. A new railway will link the holy sites around Mecca.
Reporting by David Cutler, London Editorial Reference Unit; Editing by Alison Williams