Title: Positive Hall coefficients: not the hole story (hole should be spelt hole, not whole) Abstract: The Hall coefficient measures the voltage induced across a rectanglar bar when a current flows through the bar and a magnetic field is applied perpendicular to the bar. The electric field generated by the Hall voltage counteracts t he force on the moving charge carrier due to the magnetic field. Positive Hall coefficients are usually associated with materials where negatively charged electrons are the predominant carrier, whereas negative Hall coefficients are usually associated with materials, such as doped semiconductors, where positively charged holes are the predominant carrier. However we show here that this is not the complete story. We find it is possible to obtain a material with a negative Hall coefficient by combining materials with positive Hall coefficients in an appropriate microstructure. Also we show it is possible to design microstructures where the effective Hall coefficient is much larger than the coefficient in any of the constituent phases. The situation is reminiscent of similar behavior for the effective thermal expansion coefficient, found by Lakes, Sigmund and Torquato.