You are right, If inductors are in series this wont mean they are 90° of each other...

I will try to explain to you...

If you have two coupled inductors with same inductance, and you connect them in series, there is two possibilities, or you connect them adding the fields, in this case the inductance value will be 4x greater, or you subtract and in this case you almost kill all the inductance of both coils, in this case it will not become zero because not aways all the fields are completely coupled.

-this means that in the first case the voltages of both coils adds up... In the second case the voltages cancel each other and sums to zero...

Your question, is the question of many here...

90° what it is?

An inductor is known as a mean to make voltage lead current... Now what this mean... If you apply a voltage in a circuit having an inductor in series, across the inductor initially there will be the applied volts as long as the current start flowing a voltage opposite to that applied appears across the inductor canceling the applied voltage.

If you apply this voltage across the inductor, first you will get zero current and when the current reach its maximum the voltage should become zero...

So Voltage leads current. Inductors have their higher voltage across its leads when there is a maximum rate of change in current...

The amount of phase° it will give to a signal will depends on how big is the inductor, the frequency and impedance of the circuit...

But if you get two coupled coils they can only have two configurations possible, or 0° (positive cosine) Adding, or 180° (negative cosine) subtracting...is scalar product...

At resonance at the inductor the voltage leads current by 90°... this is correct... and in the capacitor the current leads voltage 90°, because it will reach its maximum voltage when current is zero or finally stops in one direction. This is cross product...

If you have a tapped transformer, and you get the coils in the same direction, you get 0° between its output leads or maximum voltage... If the secondaries were subtracting than you get the signals 180° or zero volts out... This don't mean there is zero volts across the secondaries, the volts still there but both secondaries ends in this case are at the same time or positive or negative. so no volts between the leads... only alternating potential...

I could be confusing you on the phase stuff and i can be wrong in definition, so i tell you the definition its not even relevant, the important is to understand what is happening...

If i understood well tony was saying the 180 sums and 0° subtracts... not sure, for me depends on what you are talking about... hwr he might be right...

Try to get a book... you will see its not a 7 head beast.

To be able to understand what stan did you get to know more than he knew about this stuff so remains no doubt to you and you can go on further.