For a large power reactor it appears possible to describe nonseparable space-time kinetics transients in terms of a particular set of spatial harmonics to be called inhour modes. These modes are defined as a subset of the period modes obtained by assuming a separable time variation ewt for all variables in the source-free, time-dependent neutron and neutron-precursor equations. Their use is appropriate whenever details of the neutron energy and angular behavior are not required. Inhour modes are shown to occur in clusters of seven, the seven eigenvalues of a given cluster being obtained as the roots of an inhour equation appropriate to the cluster. The neutron flux shapes associated with a particular cluster of seven modes are all approximately the same. It is shown that if these shapes are assumed to be identical, certain useful orthogonality relations and certain identities involving the roots of the inhour formula for a given cluster are obtained. Use of these results simplifies the extension of the conventional equations of reactor kinetics to the nonseparable case. Inhour modes are also useful in analyzing certain experiments involving subcritical assemblies. As an illustration, application to the source-jerk and pulsed-source experiments is made.