what are the laws for variables in here

[transgalactic]

https://i50.tinypic.com/s3iz4j.jpg

in the top circuit they want to find northon/thevenin

and as in the bottom circuit there is no independent sources
so they add a test source to know how much current go threw it

i notices that on the top circuit we have 2 intersection
and they want to find the potential on them (v1 and v2)

but on the bottom circuit we have 3 intersections and we put
two variables instead of three .

why??
in this kind of thing
in what intersections do i need to find there potential
??
why they a KCL only on two of the intersection instead of the three present?

 

[Allobro2]

In the first circuit they must indicate i(delta) on the circuit! and then you will have the third equation, after that it will be easy to continue.

 

[mbarazeen]

in both cases you have two nodes only with unknown voltages, so maked as V1 & V2. voltages are maked on nodes that are unknown, in the 2nd diagram the said 3rd intersection(node) is having the source voltage.