Geophysics students taking their first seismic data processing course during their freshman year often wonder how we seem to be perfectly happy with single-component data in a three-dimensional world, the single component being either vertical geophones on land or hydrophones in water. Some students ask the question but then are happy to accept an answer that multicomponent is not covered in the freshman course. They quickly get busy learning what material is in the course and will be on the final exam. Deconvolution, NMO, and migration — all applied to single-component data — will determine their grade. Childhood innocence lost! Years later, sometimes at an SEG honorary lecture, it comes back. It is a relief. They always knew that one-component data could not be complete. They are glad that somebody else cares. It is then usually assumed that in a three-dimensional world, three-component data would be complete. Right? No. Seabed systems record four components, not just three. Why do we need four components in a 3D world? If four components are more complete than 3C, then would 5C be more complete than 4C? How about 6C? Where will we stop … at 7C?