Pro rata rights are the contractual right that lets an existing investor maintain ownership percentage by buying their proportional share of any future financing round. The investor exercises at the same price and terms as new investors in that round. They're standard in venture term sheets and Y Combinator SAFE side letters, and they exist so early investors can stay at their original ownership level through subsequent dilution if they choose to.
The mechanic, with numbers:
Investor A holds 5% post-seed and has full pro rata rights. The company raises a $20M Series A. Investor A's pro rata entitlement is 5% × $20M = $1M of the round. If Investor A: