What is the Ka'ba and why it is pivotal to the Islamic world? Why do pilgrims go about it, not in it? Is it empty? And why is a hollow building covered in black silk? The most sacred site of Islam, the Ka'ba (the granite cuboid structure at the centre of the Great Mosque of Mecca) is here investigated by examining six of its predominantly spatial effects: as the qibla (the direction faced in prayer); as the axis and matrix mundi of the Islamic world; as an architectural principle in the bedrock of this world; as a circumambulated goal of pilgrimage and site of spiritual union for mystics and Sufis; and as a dwelling that is imagined to shelter temporarily an animating force; but which otherwise, as a house, holds a void.