We show how the Onsager algebra, used in the original solution of the two-dimensional Ising model, arises as an infinite-dimensional symmetry of certain self-dual models that also have a U (1) symmetry. We describe in detail the example of nearest-neighbour n-state clock chains whose Z n symmetry is enhanced to U (1). As a consequence of the Onsager-algebra symmetry, the spectrum of these models possesses degeneracies with multiplicities 2 N for positive integer N . We construct the elements of the algebra explicitly from transfer matrices built from nonfundamental representations of the quantum-group algebra U q (sl 2 ). We analyse the spectra further by using both the coordinate Bethe ansatz and a functional approach, and show that the degeneracies result from special exact n-string solutions of the Bethe equations. We also find a family of commuting chiral Hamiltonians that break the degeneracies and allow an integrable interpolation between ferro-and antiferromagnets. arXiv:1812.09091v2 [cond-mat.stat-mech] 21 Jun 2019Jordan-Wigner transformation [4]. This fermionic method is even easier, and so for the most part the Onsager algebra itself was no longer exploited. Moreover, the elements of the Onsager algebra in the Ising model are all free-fermion bilinears, and their commutation relations are nice because any commutator of such fermion bilinears yields a linear combination of bilinears.It thus seemed sensible to expect that the Onsager algebra is merely one of the many marvellous properties of free-fermionic systems, and so only occurs in such. This expectation, however, is simply wrong. Motivated by some curious observations by Howes, Kadanoff and den Nijs [5], von Gehlen and Rittenberg made the remarkable observation that the Onsager algebra is obeyed by operators in an n-state clock model [6]. They then show that the algebra allows construction of an infinite series of commuting local conserved charges, strongly suggesting a certain chiral clock model commuting with them is integrable. 2 This model is indeed integrable, and now bears the name of the superintegrable chiral Potts model. The integrability allows many of its properties to be computed [9], but the Onsager algebra is not heavily utilised in this analysis.The Onsager algebra cannot be used to solve chiral clock models directly because its elements here do not have the simple periodicity property that the Ising presentation has. Thus what is free-fermionic about Onsager's original solution is the periodicity property, not the algebra itself. However, while progress has been made in understanding how the Onsager algebra relates to more standard approaches to integrability (see [10] and references therein), the question remains: what more can the Onsager algebra tell us about properties of clock models?The purpose of this paper is to define and analyse a series of clock models that have the Onsager algebra as a symmetry algebra: all its elements commute with the Hamiltonian and transfer matrix. We believe this is the simplest set of such models. W...