| Bottom |
| Reason |
| Essence |
| Grounds |
| Footing |
| Principle |
| Reasoning |
| Rationale |
| Groundwork |
| Cornerstone |
| Foundation. |
| Underpinning |
| Justification |
| Underpinnings |
| Starting point |
| Main principle |
| Presupposition |
| First principle |
| Main ingredient |
| Chief ingredient |
| Need-to-know ___ |
| Idea's foundation |
| Logical rationale |
| Underlying premise |
| Principal component |
| On a first-name ___ |
| Underlying principle |
| Fundamental principle |
| Principal constituent |
| On a need-to-know ___ |
| Cost or trial follower |
| Foundation for something |
| Foundation of an argument |
| Discussion's starting point |
| Disco Biscuits "___ for a Day |
| Foundation, as of an argument |
| Demos: Heard on a need-to-know ___ |
| Part of a capital-gain calculation |
| Amount from which to figure sale profit |
| Fire Party song about the foundation of something? |