Abstract: The apparatus of this invention is a composite combining prism for combining the beams of a ring laser gyro. It uses a composite prism whose trunk is fabricated of two substantially identical juxtaposed sub-prisms having beam splitters in the region where laser beams cross their common boundary. The beam splitter regions are very thin dielectric films, typically no thicker than a fraction of a wavelength of the laser light. Dielectric films, such as titanium dioxide, are used for the splitters so that the geometrical path lengths of interfering beams are almost exactly the same. To make the path lengths optically the same, the two sub-prisms and the substrate of the partly transmitting corner mirror of the ring laser preferably have the same index of refraction. In a first embodiment, the outwardly-extending trunk of the assembled composite prism is substantially rectangular in cross-section.