Despite the high standard of workmanship employed in its fabrication, a propeller, like any other manufactured article, invariably differs from the ideal product in some respect which may, or may not, be of functional significance according to the nature of the disparity. The component blades of a propeller are no exception to this rule and it is the author's object to examine the consequences of dimensional deviations of the actual blades from those of an assumed standard and to describe apparatus devised to minimise or eliminate the most undesirable of them, namely, the tendency to induce vibration in the aircraft. From the earliest day, aircraft vibration has been attributed to the engine propeller combination and has come to be regarded as an inevitable characteristic. It can be demonstrated, however, that where aerodynamically balanced blades are employed, vibration difficulties are not an inherent property of the propeller. This is a matter of considerable importance to the turbo-propeller in which the prime mover is virtually vibrationless and consequently any unbalance of the propeller would be thrown into prominence.