Series connection: springs experience the same force but extensions add. $1/k_{\text{eff}} = 1/k_1 + 1/k_2$. The effective constant is always less than the smallest individual constant. Parallel connection: springs share the force but have the same extension.$k_{\text{eff}} = k_1 + k_2$. The effective constant is always greater than the largest individual constant. Analogy: series springs behave like parallel resistors (reciprocal addition), and parallel springs like series resistors (direct addition). This analogy often helps avoid sign confusion.$