To the east, the Red Sea defines the boundary between Africa and Asia, and thus also between Arab Africa and Arab Western Asia. This sea is a long and narrow waterway with a northwest tilt, stretching 2,300 kilometers from Egypt's Sinai peninsula southeast to the Bab-el-Mandeb strait between Djibouti in Africa and Yemen in Arabia but on average just 150 kilometers wide. Though the sea is navigable along its length, historically much contact between Arab Africa and Arab Western Asia has been either overland across the Sinai or by sea across the Mediterranean or the narrow Bab al Mendeb strait. From northwest to southeast, Egypt, Sudan, and Eritrea form the African coastline, with Djibouti marking Bab al Mendeb's African shore.