What is the grandfathering clause in LTCG tax for equity investments?

Equity gains accrued up to January 31, 2018 are exempt from LTCG tax — this is the grandfathering provision. Cost for tax purposes is the higher of actual purchase price or the market price on Jan 31, 2018. Only gains made after that date are subject to the 10% (now 12.5%) LTCG tax.