The mid flight treads may vary in shape and size, different treads have different purposes, and a combination of the different treads in one flight is quite normal.
Treads of one style may vary in size throughout the flight.
The straight flier is the most common of treads, this may be used in any straight run of treads.
These will be constant in both rise and going throughout the entire flight.
The treads may vary in rise and going on different flights; to allow for the difference in space between the different finished floor levels.
The diminishing flier is an unsung hero of the staircase world.
This tread is used to change the pitch angle of the well string before the springing point of the turn, this is a great advantage when setting out the handrail; as it will divorce the change in pitch from the change in direction, making the handrail flow much smoother.
The winder tread is used to change direction in a flight of straight fliers, the tread length at the walking line will be similar to the going on the straight fliers.
The winder may be used in straight flights that change direction.
Elliptical stairs may be created using a full flight of various size winders.
A spiral flight will be made from constant size winders.
Kite winders are as one would suspect, treads that are kite shaped, these will go from the well string into the corner of a wall string.
The kite winders will have winders on either side and will normally be set in the corner of a 90º turn.