Bandstop Filters With Extended Upper Passbands

I have a question on Bandstop Filters With Extended Upper Passbands

What does it exactly mean by θ ?

Look on this:
https://en.wikipedia.org/wiki/Commensurate_line_circuit

But that wikipedia link does not mention about θ ?

Please correct me if I miss anything.

As in any well written scientific paper, terms are introduced in the text. θ is electrical length of a line segment, θr electrical length at resonance (center frequency).