The maritime boundary between Equatorial Guinea and São Tomé and Príncipe (STP) is comprised of two separate boundary segments, one in the north between STP and the mainland of Equatorial Guinea and its principal island Bioko, and the other in the south between STP and the more diminutive Annobón Island. The northern portion of the boundary is defined by 15 coordinate points and extends for 193 M, while the southern portion is defined by 5 coordinate points and extends for 284 M. These boundaries were established through a bilateral agreement that was reached in 1999 and largely formulated using the equidistance methodology.