Apparently, the angle, θ, between the strings is important and we haven't discussed why, so let's have a look at that.
Figure 3 (3a) (3b): In figure 3, you see two pairs of pith balls hanging from strings. Beside each ball is a dot where you'll draw force vectors to create a free body diagram (FBD). In each FBD, the balls will be in equilibrium. This means that ΣFx = 0 and ΣFy = 0. In figure 3a, the pith balls are side-by-side and the strings are vertical. (This is impossible with this apparatus since the strings are attached at a point, but that won't be an issue since we aren't concerned with their behavior at this angle.) The vertical orientation of the strings could indicate one of two things. The balls could be oppositely charged. But we'll never see that case since our pith balls can only be neutral or negatively charged.