This expression is valid for all times for semi-infinite bodies in perfect thermal contact.

If two semi-infinite bodies initially at temperatures T and T are brought in perfect thermal contact, the temperature at the contact surface T will be given by their relative effusivities.

The zeroth boundary condition is important for identifying the presence of a coordinate boundary where no physical boundary exists, for example, far away in a semi-infinite body or at the center of a cylindrical or spherical body.