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We present a practical method for measuring planar gaps of microelectromechanical systems (MEMS) with comb drives by on- or off-chip electrical probing. We show that our method is practical, accurate, precise, and repeatable. The option of on-chip postpackaged electrical measurement enables MEMS to be autonomously self-calibratable. We use the measurement of gap to determine the geometrical difference between layout and fabrication, which can lead to measurements of other properties such as displacement, force, stiffness, and mass. Our method consists of applying enough voltage to close two unequal gaps and measuring the resulting changes in capacitances. Many MEMS designs with comb drives can be easily modified to implement our technology. Our results are an order better than convention and suggest means for further improvement.