When equity gains became taxable in 2018, the government "grandfathered" existing gains—your cost is stepped up to the January 31, 2018 value, protecting older appreciation. Many investors calculate gains from original purchase price, overpaying tax on decades-old holdings. Grandfathering is legislative mercy, but only if you apply it correctly. History has value the law recognizes. Do you compute your equity gains using the grandfathered value or your original cost?