To correctly install a new single coil pickup in your Strat along with pickups you already have, you need to understand the winding direction and pole piece polarity orientation of each of your pickups before installing them together to avoid the possibility of phasing issues.

For all 3 single coil pickups in a Strat to be in phase with one another when two pickups are selected together all 3 of your pickups must be wound the same direction and have the same pole piece polarity orientation and for Strat's that have hum cancelling capabilities in selector switch positions 2 and 4, the middle pickup needs to be the complete opposite (winding and polarity) of the bridge and neck pickups which means the middle pickup needs to be wound in the opposite direction and have the pole pieces polarity orientated the opposite direction, this is what is known as Reverse Wound / Reverse Polarity (Rw/Rp).  A Reverse Wound / Reverse Polarity pickup is only in reference to the other two pickups in an installation this is because one pickup manufacturer's Rw/Rp pickup can be another manufacturer's normal wind / normal polarity. There is no standard for how single coil pickups are wound and their pole pieces polarity orientation.

So if you are planning on upgrading your Strat with a new pickup, you need to know the winding direction and pole piece polarity orientation of the new pickup as well as your other two existing pickups to avoid phasing issues.

How A Pickups Winding Direction Can Be Determined

Below is a single coil pickup that is wound Counter Clock-Wise which can easily be identified by a simple visual inspection. Shown below with the Red Arrow is the finish wrap of the pickups winding wire as it comes off the coil.  This is what is will show you the winding direction.  In looking at the photograph you can see how the small copper wire that connects to the white hook-up lead is coming off of the bottom left side which is from the outer-most or final wrap of the pickups coil. This indicates that the winding direction was counter clock-wise. You will also notice the small copper coil winding wire that connects the the black hook-up lead tucks well underneath the windings of the coil on the right side, this indicates that it was the very beginning of the coil winding and subsequent winding wraps built up over it leaving it tucked well underneath.

How A Pickups Pole Pieces Polarity Orientation Can Be Determined

Below is a single coil pickup that has is pole pieces polarity orientated North Up. This pickup construction characteristic can easily be determined by placing a magnetic compass over the pole pieces as shown below. The magnetic pole pieces of the pickup will attract the opposite magnetic polarity end of the compass needle which in the example below is South (white). You can look at it like this; the pole piece polarity of the pickup is the end of the needle that is pointing away from the pickup, and in this case it is the red end of the needle which is North. This pickup is North up for its pole pieces.

Below Are Some Examples Of How Current Production Fender And Seymour Duncan True Single Coil Pickups Are Wound