When a dielectric fills only part of the gap between parallel plates, model it as capacitors in series or parallel. Dielectric of thickness t (t < d) filling the full area: two capacitors in series — C_eq = epsilon_0A / (d - t + t/K). Dielectric filling half the area (full thickness): two capacitors in parallel — C_eq = epsilon_0A*(1 + K)/(2d). Multiple layers of different dielectrics: series if layers are perpendicular to E (stacked), parallel if they divide the area. For a conducting slab of thickness t: K -> infinity, so C = epsilon_0*A/(d-t). A conducting slab of thickness t is equivalent to reducing the gap by t.