We describe a new class of propagation-invariant light beams with Fourier transform given by an eigenfunction of the quantum mechanical pendulum. These beams, whose spectra (restricted to a circle) are doubly-periodic Mathieu functions in azimuth, depend on a field strength parameter. When the parameter is zero, pendulum beams are Bessel beams, and as the parameter approaches infinity, they resemble transversely propagating one-dimensional Gaussian wavepackets (Gaussian beam-beams). Pendulum beams are the eigenfunctions of an operator which interpolates between the squared angular momentum operator and the linear momentum operator. The analysis reveals connections with Mathieu beams, and insight into the paraxial approximation. Special properties of the functions are often related to the physical problems in which they are typically studied; for instance, higher-order Gaussian beams in the focal plane arise as eigenfunctions of the twodimensional harmonic oscillator [10], both as HermiteGauss (HG) modes (quantized back-and-forth harmonic motion), and Laguerre-Gauss (LG) modes (quantized circular harmonic motion) [6]. The quantum states corresponding to elliptic vibration states are the 'Generalized Hermite-Laguerre Gaussian' (GG) beams [11], which provide a smooth interpolation of higher-order Gaussian profiles between linear, standing wave-like HG modes and rotating, OAM-carrying LG modes, naturally related to quantum angular momentum algebra [10].Here, we consider a nondiffracting beam family counterpart to GG beams, interpolating between standing azimuthal Bessel beams and travelling plane waves with a transverse momentum component. The family is based on the set of special function solutions of a wellunderstood physical problem, namely the eigenstates of a quantum pendulum [12,13] for a matter wave confined to a circle of radius L subject to a constant gravitational field strength g, which acts as the interpolating parameter for 0 ≤ g ≤ ∞. Such pendulum eigenstates are complex distributions on the circle with angle θ (with radius the pendulum length L), which we consider as the spectral distribution of our beams in Fourier space.With a spectrum restricted to the circle (with L → k r , the radial wavenumber), the propagating beam in real space has an invariant amplitude profile upon propagation [1,8]. The eigenfunctions of the quantum pendulum, first studied by Condon [12] and subsequently re-discovered several times [13] are Mathieu functions [13,14], which determine the spectra of our 'pendulum beams'. The ground state of the pendulum has been considered as a beam spectrum previously [15], minimizing a certain angular uncertainty relation. Our definition of this new beam family reveals interesting aspects of optical linear and orbital angular momentum, extends the GG beam interpolation to propagation-invariant beams, and provides an unusual appearance of the paraxial approximation. Furthermore, pendulum beams arise naturally as eigenfunctions of a certain operator which interpolates between angular and lin...