In a transmission line, signal velocity is the reciprocal of the square root of the capacitance-inductance product, where inductance and capacitance are typically expressed as per-unit length. In circuit boards made of FR-4 material, the signal velocity is typically about six inches (15 cm) per nanosecond, or 6.562 ps/mm. In circuit boards made of Polyimide material, the signal velocity is typically about 16.3 cm per nanosecond or 6.146 ps/mm. In these boards, permeability is usually constant and dielectric constant often varies from location to location, causing variations in signal velocity. As data rates increase, these variations become a major concern for computer manufacturers.