I'm learning from Paul Scherz's *Practical Electronics for Inventors* (2nd ed.) and I've become confused at what was I thought I was comfortable with: namely parallel and series resistors. The offending example is:

![Worked example of Thévenin's Theorem](https://i.stack.imgur.com/B4p8W.jpg)

Looking at the uppermost diagram and working out the simple Thévenin equivalent circuit for the circled loop, I don't understand why the total Thévenin resistance is the sum of individual *parallel* resistors and not *series* resistors. 

My reasoning tells me that since the other half of the circuit has been shorted, then an equivalent current flows through both resistors in this loop i.e. implying a series arrangement. Going further, even if the circuit were complete, these two resistors would still be in series, so I'm confused as to why the Thévenin resistance has been calculated as if this is a parallel arrangement.