Rankine's earth pressure theory quantifies the lateral thrust exerted by backfill soil against a retaining structure as the wall yields outward. This lateral yield mobilizes the internal friction angle of the granular backfill along a planar failure wedge inclined at 45° + φ/2 to the horizontal. The active horizontal earth pressure increases linearly with depth, producing a triangular stress distribution whose resultant acts at one-third the wall height.