Single fins are more commonly used in Longboards and Mid Length Boards. They are still highly popular as the larger fin provides excellent drive in small conditions but the right surfer can surf them in almost anything. Demonstrated by legends such as Gerry Lopez, Mark Richards and Michael Peterson who surfed these boards in waves of consequence during the golden era of the '70s.
There are multiple different styles of Single Fins such as the Flex Fin, Rake Fin, Pivot Fin and D Fin.
The Flex Fin (maximum flex) is a very versatile fin that can be used in everything from short stubbies and mid length hulls, to 10 foot nose riders. They usually feature a wider base, tapering down to a narrow tip and offer very little drag and a lot of projection when coming off a bottom turn. The True Ames Greenough single fin is the original flex fin and it has been loved for decades.
The Rake Fin (medium flex) kind of splits the difference between the Pivot and the Flex fins. A wide base but with more tip than the flex fin, and usually starting out fairly vertical then sweeping back a lot towards the end. These provide more projection and manoeuvrability than a Pivot fin, but still allow you to get the tail to hold when you are up on the nose.
The Pivot Fin (very little flex) is the standard go to template for nose riding. The feature a full vertical outline, wide base and are designed to slow you down and help keep the tail in the water while you are up on the nose, yet still allow you to step on the tail and swing (pivot) the board around in a relatively tight radius.
The D Fin (no flex) is not the most versatile or forgiving fin to surf. Usually found on more classic, heavy logs, they are harder to turn, and a little less stable while up on the nose. If you are looking for maximum trim straight down the line, however, the D Fin is for you.
Check out the Single Fin range from brands such as FCS, Futures, Captain Fin Co and True Ames.