In a lot of historical romance novels (specifically those set in Regency England), people decide to get married very suddenly a lot of the time. You see this a lot in the marriage of convenience trope, but also in various other flavours of historical romance.
When this happens, they usually either end up running away to Gretna Green (because apparently anyone there can witness/perform marriages, at least in the era historical romances are usually set) or getting a "special licence" that allows them to get married very quickly.
What's this special licence? Why is it needed? Is this an actual thing that was required of people of the time, or is it merely a romance novel trope that allows the author to sidestep the actual laws/rules of marriage in that time period?