Converging Verticals
The apparent leaning-together of vertical lines when the camera is tilted up or down.
In depth
Verticals stay parallel only when the sensor is parallel to them, so any tilt introduces convergence. The remedies are to keep the camera level and crop, to shift a tilt-shift lens upward, or to correct in software at the cost of resolution. Left uncorrected on a building, the effect reads as a mistake; used deliberately from ground level it reads as drama.
Examples in practice
Tilting up at a tower block makes its sides lean inward until a shift lens or a perspective correction restores them.