River terraces are classified as paired or unpaired depending on the style of base-level change and channel behavior during incision: paired terraces form from episodic (cyclic) base-level fall combined with a channel that stays laterally fixed (non-migrating) during each incision event, leaving symmetric terrace remnants at matching elevations on both valley sides; unpaired (non-cyclic) terraces, by contrast, require a specific combination of base-level and channel behavior, and any scenario lacking the necessary base-level fall component (i.e., involving a base-level rise rather than fall) cannot produce terracing at all, regardless of channel migration behavior.