The evolution of higher order {221} and {331} crystal planes during corner undercutting in the anisotropic etching of (100) silicon is discussed, and the occurrence of highly vertical (72.5 degrees) {311} planes unique to KOH etches are demonstrated. Using a combined etching technique, very high aspect ratio micro-tips are formed and their distinct advantages for vacuum microelectronics and field-emission devices (FED) are described.