Ab initio calculations were combined with anion photoelectron spectroscopy to unravel the structural origin of Al-5 and Al-5(-). Well-resolved photoelectron spectra of Al-5(-) were obtained and compared to theoretical calculations performed at various levels of theory. It was shown that the best agreement between the experimental and theoretical data is for a planar C-2v structure. Analyses of the electronic structure and molecular orbitals revealed that the planarity in Al-5 and Al-5(-) are due to the presence of a four-center peripheral bond that is common in a whole family of planar pentaatomic species recently uncovered.