Soap makers have known for a very long time that the consistency of a soap and water mixture depends very strongly on its composition. From fluid and optically isotropic when it is rich in water (micellar phase), the mixture suddenly becomes very viscous when the soap concentration exceeds 40 to 50% by weight. This new phase, birefringent and very pasty, was first named “middle phase,” as it always appears in the center of the phase diagrams. X-ray studies showed that the amphiphilic molecules form tubular aggregates (or columns) that organize on a hexagonal lattice, hence the name hexagonal phase (denoted by H1 in the literature) (Fig. C.IX.1).