Example 2
Rich and Yvette are excited about their new home; they're less excited about paying down their mortgage principal.
Here, principal refers to a sum of money. Maybe Rich and Yvette should hold a cash-only housewarming party. Sure, it's tasteless, but a fat stack of $100 bills is far more useful than some wine charms or a magazine rack.
Example 3
For seventy-five years, my grandma has stuck to her principles and never, ever worn white after Labor Day.
In this fashion-conscious sentence, principles is the correct word because it refers to a set of rules that Grandma has set for herself.