Rules on hyphenation:

• When used as adjectives, hyphenate different orders: first-order, second-order, low-order, high-order. Note however that a low-order model is low order (no hyphen when order is the noun).
• For reasons that I don't understand, the term "spring-mass" is hyphenated using an en-dash: spring--mass. However, it is indexed with a single dash (spring-mass).
• Hyphenate steady-state when used as an adjective (steady-state solution), but not when used as a noun (the system reaches steady state).
• We don't hyphenate open loop and closed loop, even when used as adjectives.

A list of hyphenated words from the index: TBD

Related topic: combination works

• Use pole/zero in phrases like "pole/zero cancellation". However, we write "pole zero map" (for MATLAB pzmap).