In a situation where data are known to three significant digits, we write 6.379 m = 6.38 m and 6.374 m = 6.37m. When a number ends in 5, we arbitrarily choose to write 6.375 m = 6.38m. We could equally well write 6.375 m = 6.37 m, “rounding down” instead of “rounding up,” because we would change the number 6.375 by equal increments in both cases. Now consider an order-of-magnitude estimate, in which we consider factors rather than increments. We write 500 m ~ 103 m because 500 differs from 100 by a factor of 5 while it differs from 1 000 by only a factor of 2. We write 437 m ~ 103 m and 305 m ~ 102 m. What distance differs from 100 m and from 1 000 m by equal factors, so that we could equally well choose to represent its order of magnitude either as ~102 m or as ~103 m?