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The use of orthogonal lead recordings to characterize signal-averaged late potentials is discussed. A theoretical analysis is made of the performances of individual orthogonal leads and their vector magnitude. Some limitations of the vector magnitude for characterizing late potentials are developed in relation to the distribution of signal and noise among its component orthogonal leads. Starting from the basic statistical assumptions required for signal averaging, an analysis of late potentials was simulated using ideal signal and noise functions. The results from theory and practice show an ability to predict the relative performances of signal-averaged orthogonal leads and their vector magnitude under typical recording conditions. This is illustrated with some observations from patient data. The conclusion of this investigation is that the vector magnitude waveform is not always able to reflect accurately the original information available in the signal-averaged orthogonal leads.