A downward change in the conversion price (or exercise price) of a convertible security. For a full ratchet antidilution clause, the conversion price of the convertible security is reduced to the exact price at which any subsequent security of the issuer is sold at a lower price, regardless of the amount of subsequent securities sold. If an investor purchased convertible preferred stock that is initially convertible at $5 per share and the company subsequently sells even a single share of stock (common stock or preferred stock) at $2 per share, then, as a result of applying the full ratchet, the investor has the right to convert the investor's own convertible preferred stock at $2 per share rather than $5 per share. A full ratchet is generally not available if the stock purchased at a lower price is sold under the company's stock option plan.