Challenges in energy storage, electronics, and catalysis motivate the search for exotic materials with extreme properties, and electrides-crystals with bare electrons trapped at stoichiometric concentrations -offer some of the most exceptional. These electrons have been ejected from atomic orbitals to reside in vacant lattice sites and, because they are so weakly bound, are better electron donors than alkali metals , can offer electrical conductivity that rivals silver, and can catalyze challenging reactions. These properties have led to the exploration of electrides in applications where electron-rich materials are needed: N2 and CO2 reduction , battery electrodes , and electron emitters . Despite this progress, rules that might predict an electride's properties based on its structure or composition are underdeveloped. For example, unlike in conventional materials, it is unknown how to tune the band gap of semiconducting electrides. And, even more critically, simple rules that could predict whether a material is an electride remain imprecise .