If we wish to preserve in DeepSMILES the hydrogen-suppressed nature of SMILES strings (see Discussion for alternative approaches), it is not possible to achieve an entirely parenthesis free syntax. However, only a single parenthesis is required instead of the two in regular SMILES. The order of atoms in a SMILES string represents a tree traversal of the underlying molecular graph. DeepSMILES preserves the same traversal order but indicates moving to a new branch using parentheses, where the number of parentheses indicates how far back on the current branch the new branch begins. Where no parentheses appear, it means that the next atom is attached directly to the previous atom, while a single parenthesis means that it is instead attached to the second-last atom on the current branch.