Two families of friction law are classically introduced to describe mechanical fault instabilities: slip weakening and rate and state friction laws. Generally opposed, we propose here, on the basis of our experimental results, to combine them in a single unified law. Using a large displacement ring shear apparatus for thick gouge samples (confinement: 0.1 - 0.5 MPa), we observe that slip weakening largely dominates rate and state effects. The effective friction coefficient is found to undergo a powerlaw decrease with imposed slip. Although no characteristic length scale really exists, the main decrease of effective friction occurs over about 50cm of slip. This appears quantitatively consistent with seismological data both in terms of typical weakening distances and characteristic rupture energies. Rate and state effects are involved over significantly smaller scales: d_c about 100 microns. On the microscale, grain attrition exists. An image correlation technique reveals that the active interface is significantly wider than suggested by the microstructures. Slip weakening appears caused by a progressive mechanical decoupling between a shear band and the bulk of the sample. Direct normal stress measurements along the shear interface have been possible and show a negligible influence of hoop stresses. Implications for faults are finally discussed.