Keys are used to connect two coaxial shafts to transmit power. There are several types of keys including sunk keys, saddle keys, tangent keys, and round keys. Sunk keys like rectangular, square, parallel, gib-head, and feather keys are common. Woodruff keys allow for easy adjustment. The key length must be sufficient to transmit the full torque of the shaft based on shear strength calculations. A keyway weakens the shaft so its dimensions are important. Splined shafts can transmit more power than a single keyed shaft.