Inductance of a double circuit line -- struggling with return the path

[jaus tail]

So for inductance of transmission line book says formula
H = 2 * 10 ^-7 Log_e { [Ds]/[Dm] }
I'm struggling to understand self and mutual geometric mean distance.
Here is one solved example

and for this they've drawn

And they say that let a a' form one connection and let b b' form other connection.
I'm struggling to understand how are they in parallel.
Current will go from a and return from a' or return from b?
How can b and b' be return path? As per question two upper and two lower conductors are in parallel so it means a and b are in parallel. So current in a and current in b have to be same.

And in three phase there are conductors r - r', y - y', b - b' So does this mean r and r' are in parallel or is it like current goes from r and returns from r' at one instant.

 

[jaus tail]

This 3-phase double circuit.

I don't understand what does a -a' mean. Are they in parallel or do they carry currents in opposite direction?
And here is double circuit another example.

 

[Tom.G]

From the text description I get.

In the first image. You are looking at the ends of four wires. They extend into the page. It seems the current requirement is greater than is convenient for a single conductor to carry. To handle the extra current, two conductors are used, wired electrically in parallel. A set of two conductors would be A&B (call that pair the 'supply' side). For the 'return' side the set of two conductors is A'&B'.

That is certainly not the way I would prefer to label them, it makes it too confusing... perhaps that is the 'standard' way of labeling them, but it seems strange.

p.s. If you are graded as 'wrong', you may have to point out the ambiguous description to your instructor. Print this out if needed to show him.

 

[jaus tail]

Thanks. So according to you the top conductors carry current I and bottom conductors carry -I at a given instant of time?