The orbital interaction between the sheets of graphite slabs, in which small numbers of sheets (n) are stacked in the ABAB fashion, is theoretically analyzed. We predict from the nodal properties in the interlayer orbital interaction that the spacing between the graphite sheets should be large (small) when it is even (odd). Results from density functional theory calculations with gradient corrections are in good agreement with this theoretical prediction; optimized interlayer spacings show oscillatory behavior as a function of n. The interlayer spacing is predicted to be 3.58 and 3.30 Angstrom when n is 2 and 3, respectively. The spacing in the double-layer slab is surprisingly large in comparison with the value of 3.35 Angstrom observed in natural graphite. The significant features of the interlayer spacing in the graphite slabs are discussed in detail, with an emphasis on the aspect of frontier orbital ideas and orbital symmetry concepts.