A block code is presented that will correct an error consisting of the gain or loss of a bit (binary digit) within the block. The code can be generalized to correct the loss or gain of a burst of bits. A further feature is the possibility of correcting additive errors appearing in the vicinity of the bit loss or gain. An additive error is a bit changed from 0 to 1 or from 1 to 0. The code is constructed by inserting a known character into a burst-error-correcting code at periodic intervals. The known character locates the approximate position of the bit loss or gain. At the location a bit is inserted or removed from the block, depending on whether a loss or a gain has occurred. The error-correcting code then corrects the erroneous bits between where the error occurred and where the correction took place.