Although deck stairs seem to come in many different styles there are basically only two types in common use. The reason is really safety. A deck can be quite a height from ground level and nobody wants friends falling off ill considered stairs.
Deck steps built on serrated stringers are supported on the edge of the stringer that has been deliberately cut to do so. The serrations on the stringer are designed so that, when the stringer is in position against the deck, they will be horizontal and vertical. The horizontal edge of the serration supports the step and the vertical edge supports the riser.
Mortised stringers can either have a properly rebated mortise cut into them or have the mortise made up by metal cleats or ‘L’ brackets. The steps and risers are fitted to the stringer by the mortise or cleats. Because the stringers have been reduce much less than the serrated stringers, mortised stringers tend to be stronger and last longer.
Either type of deck stair stringer can be taken round a 90º corner to create a landing and reduce the distance that the foot of the stairs is from the deck.