Simple estimate of the opening angle of the region around the terminator that affects the transmission spectrum (i.e. the limb). The contours give the opening angle in degrees as a function of the planetary radius and the atmospheric scale height of the atmosphere at 10 mb (the variation of gravity with height is accounted for). The atmosphere is assumed to become transparent above the 1 Pa pressure level. Black dots are known planets for which the radius and surface gravity are measured. Some key objects are identified. The equivalent temperature is derived from the scale height assuming an atmosphere of hydrogen and helium (solar abundances) and a surface gravity of 10 m s⁻². For hot, low-gravity planets like GJ 1214 b, the limb can cover almost half the surface of the planet.