The boundary begins in the Persian Gulf at the "lowest point of low water" at the mouth of the Shatt al-Arab at (WGS84.) It then closely following the thalweg of the Shatt al-Arab for some 105 kilometers in a series of short straight line segments, reaching the confluence of the Shatt al-Arab and the Nahr al-Khayin tributary. From there it winds northward, following a series of boundary markers across plain and hill, through the Zagros Mountains Nahr at-Tib, and Nahr Wadi. It meets the boundary with Turkey at 37° 08' 44" N and 44° 47' 05" E.
A more precise demarcation was begun in 1911 at the urging of Russia and Great Britain, both of whom had colonial aspirations in the region. In 1913-1914, a commission established by the Constantinople Protocol set the revised boundary, with control of the Shatt al-Arab going to Turkey. In general, the line was to follow the east bank of the waterway, except in the region surrounding the Persian town of Khorramshahr, where it was to follow the thalweg.
This was challenged by Iran in 1934, as the validity of both the Treaty of Erzerum and the Constantinople Protocol was called into question. The dispute was resolved in 1937, following the general lines of the old boundary, with the exception of the area immediately around the Iranian town of Abadan, where the boundary was moved from the east bank to the thalweg, as had been done around Khorramshahr two decades earlier.
While this resolved Iran's major grievances, it failed to respond to the issue of freedom of navigation in the Shatt al-Arab. This was resolved in 1975, when Iran and Iraq signed an agreement in Algiers in which the thalweg was determined to be the boundary throughout the Shatt al-Arab waterway. In return, Iran promised to cease its support for Kurdish rebels in northern Iraq. The new treaty was rejected by Iraq five years later, and was a cause of the Iran-Iraq war. Iraq finally accepted the new boundary in 1990 following its failed invasion of Kuwait, in accordance with United Nations United Nations Security Council Resolution 598.