Also known as a bettering right. For transactions to which the Takeover Code (www.practicallaw.com/0-107-7362) applies, topping or bettering rights given by a target are generally prohibited under Rule 21.2 of the Takeover Code. In the context of recommended bids (www.practicallaw.com/0-107-7116), a topping or bettering right, under which the target board could withdraw its recommendation, unless the bidder increased its offer to a level above a competing offer within an agreed time period, was a deal protection measure that was in frequent use before the general prohibition on offer-related arrangements was introduced into the Takeover Code on 19 September 2011. (An implementation agreement often provided a minimum amount by which the bidder had to "top" or "better" the competing offer in order to retain the target board’s recommendation.)
For examples of takeover offers where there was a topping or bettering right, see What's Market, Public M&A deals: matching or bettering right.