In simple models of passive flow beneath mid-ocean ridges, fractional melting of the upwelling mantle produces unexpected relationships amongst crustal thickness, degree of partial melting, and the average depth of melting. If the degree of melting is estimated on the basis of basalt composition, then the average degree of melting is one-third the maximum degree of melting, and the average depth of melting is about two-thirds the maximum depth in the simplest models. The crustal thickness is about 1.5 times the product of the average fraction of melting and the height of the melting column. If the average degree of melting represented in mid-ocean ridge basalts were 10% and melt production continued to the base of the crust, then only a 40-km-high residual melting column would be required to produce a 6-km-thick crust. In more realistic models, melting beginning at 60 km below the base of the crust and continuing to the top of the mantle would produce a 6-km-thick crust with average degree of melting of 6.67% or melting beginning at 67 km and ending at 30 km below the base of the crust would produce a 6-km-thick crust with an average degree of melting of only about 5.4%. Models of composition produced by fractional melting must consider the rate of melt production and the degree of melting throughout the melting region at the points where the melt is actually produced. ¿ American Geophysical Union 1993