The Arab Empire significantly contributed to navigation, and had trade networks extending from the Atlantic Ocean and Mediterranean Sea in the west to the Indian Ocean and China Sea in the east, Apart from the Nile, Tigris and Euphrates, navigable rivers in the Islamic regions were uncommon, so transport by sea was very important. Islamic geography and navigational sciences made use of a magnetic compass and a rudimentary instrument known as a kamal, used for celestial navigation and for measuring the altitudes and latitudes of the stars. The kamal itself was simple to construct. It was a rectangular piece of either bone or wood which had a string with 9 consecutive knots attached to it. Another instrument available, developed by the Arabs as well, was the quadrant. Also a celestial navigation device, it was originally developed for astronomy and later transitioned to navigation. When combined with detailed maps of the period, sailors were able to sail across oceans rather than skirt along the coast. However, there are no records of open ocean Atlantic sailing, and their activity focused on Mediterranean, Red Sea, Persian Gulf, Arabian Sea and across to the Bay of Bengal. Muslim sailors were also responsible for the use and development of the lateen sails and large three-masted merchant vessels to the Mediterranean. The origins of the caravel ship, developed and used for long-distance travel by the Portuguese, and later by the rest of Iberians, since the 15th century, also date back to the "qarib" used by Andalusian explorers by the 13th century.