  Recently, Miller et al. showed that a model's in-distribution (ID) accuracy
has a strong linear correlation with its out-of-distribution (OOD) accuracy on
several OOD benchmarks -- a phenomenon they dubbed ''accuracy-on-the-line''.
While a useful tool for model selection (i.e., the model most likely to perform
the best OOD is the one with highest ID accuracy), this fact does not help
estimate the actual OOD performance of models without access to a labeled OOD
validation set. In this paper, we show a similar but surprising phenomenon also
holds for the agreement between pairs of neural network classifiers: whenever
accuracy-on-the-line holds, we observe that the OOD agreement between the
predictions of any two pairs of neural networks (with potentially different
architectures) also observes a strong linear correlation with their ID
agreement. Furthermore, we observe that the slope and bias of OOD vs ID
agreement closely matches that of OOD vs ID accuracy. This phenomenon, which we
call agreement-on-the-line, has important practical applications: without any
labeled data, we can predict the OOD accuracy of classifiers}, since OOD
agreement can be estimated with just unlabeled data. Our prediction algorithm
outperforms previous methods both in shifts where agreement-on-the-line holds
and, surprisingly, when accuracy is not on the line. This phenomenon also
provides new insights into deep neural networks: unlike accuracy-on-the-line,
agreement-on-the-line appears to only hold for neural network classifiers.
