Triangle: three interferometers, each with a 60° angle between its arms, forming an equilateral triangle (with two parallel interferometer arms along each side) of 10 km radius

Double ell: two interferometers, each with a 90° angle between its arms, at two different sites several hundred kilometres away, with 45° angles between the arms of the two different interferometers

The triangle configuration allows three interferometers to be deployed by only boring three tunnels, and allows all three to be in the geologically optimal place; the double ell configuration requires four tunnels to be bored to deploy only two interferometers, but it would provide a better localization of events, and would allow the two sites to be in different countries, which might be politically advantageous (as each funding agency might be more willing to fund an experiment at least partly in its own country)

(Actually, the triangle configuration would comprise three pairs of interferometers parallel to each other, each with one for high frequencies and one for low frequencies, and the double ell would likewise comprise two pairs of interferometers)