Before there were computers, insurance companies used a round slide rule device called an "Insurance Wheel" to calculate how much premium was to be returned for a policy cancelled before its expiration date.
Insurance Wheel replaces the old circular slide rules.
Insurance Wheel supports the 3 normal ways of calculating how much premium is returned when an insurance policy is cancelled before its expiration date:
Pro Rata - simple fraction of unearned policy period
Short Rate (90% pro rata) - pro rata discounted 10%