A Taxonomy of Event-Linked Perpetual Futures: Design Axes, Failure Modes, and Empirical Evaluability

The label event-linked perpetual often conflates mathematically different contracts. This paper replaces a flat product list with a four-axis taxonomy spanning underlying geometry, temporal structure, settlement structure, and venue-oracle composition. It covers single binary probabilities, conditional ratios, event spreads, baskets, path functionals, liquidity indices, rolling sequences, and flow-only swaps. Six narrow results establish, among other points, that terminal shortfall depends on exact settlement support and collateral, conditional ratios become locally ill-conditioned near zero, and fixed-path replay does not identify deployment behaviour when a liquidity contract changes its own reference process. No new empirical estimates are reported. Each contract must separately define support, clocks, settlement, source hierarchy, and the controls implied by those choices.