Why are some Texas borders not straight?

Texas's northern and eastern boundaries were agreed upon when it became a state in 1845. Its southern border was set in 1850. Its western border is generally along the 103rd meridian west, set in 1850, but a correction in 1927 added a small piece known as the Oklahoma Panhandle. All are straight lines on the map, except the eastern border, which follows the Sabine River.