Aneurysms form as abnormal bulges over an artery, and, if ruptured, can lead to serious health complications or even death. Some can exist for a long time without rupturing, and surgery can be risky, so a parameter to help surgeons is needed. Researchers report that they have developed a simple nondimensional parameter that depends on both geometry and flow waveform to classify the flow mode in both sidewall and bifurcation aneurysms.