The Laws of Circuit - you can learn and practice by just reading

copyright. Charles Kim 2006

Series and Parallel Resistors
Series Resistors
If TWO (not one nor three) resistors are cascaded (or connected sequentially), they are in series. Very simple. But not easily applied by many, especially when more than 2 resistors are involved. I will give you a very convenient stick-ruler for testing any two resistors you picked are in series or not. This ruler checks just one thing: if and only if TWO (again, not one nor three) resistors are met at a node. If somehow you are not sure if only two resistors are connected at a node, check if the current in a resistor is the same as the current in the other resistor. How to check this? Apply the zero cash flow and KCL at the node.
Parallel Resistors
If 2 or more resistors are connected to the SAME TWO nodes (not one nore three), they are in parallel, and they are qualified for WMD. The key here is the term "to the SAME TWO nodes" they are all connected. Again, for some reason, you are not sure of their connectivity, check if each voltage across eithrt is the same. The voltages across the resistors in parallel must be the same.