In pymatching, it writes : rather than predicting which exact set of error mechanisms occurred, we typically want to predict the outcome of logical observable measurements, which are the parities of error mechanisms. These can be represented by a binary matrix observables. Similar to the check matrix, observables[i,j] is 1 if logical observable i is flipped by error mechanism j Also in the code for 5-qubit repetition code, it has: H = csc_matrix([[1, 1, 0, 0, 0], [0, 1, 1, 0, 0], [0, 0, 1, 1, 0]