> ## Documentation Index
> Fetch the complete documentation index at: https://docs.beliefsystems.xyz/llms.txt
> Use this file to discover all available pages before exploring further.
# Perpetual Series
> How Perpetual series differ from Fixed series – composition evolves over time through formal reconstitution, with continuous index history.
Belief Index series are classified by **Maturity Type**: Fixed or Perpetual. The distinction determines whether the series has a defined endpoint or whether its composition evolves over time.
This page is the authoritative methodology for the Perpetual class – what reconstitution is, how
often the series is reviewed, and how continuity is preserved. For the criteria candidate markets
must meet and how admitted markets are weighted, see [Market Selection &
Weighting](/indices/selection-and-weighting). For the arithmetic of chain-linking, see [Index
Calculation Methodology](/indices/nav-methodology).
## Fixed vs. Perpetual at a Glance
| Property | Fixed Series | Perpetual Series |
| ------------------------------------------- | ------------------------------------------------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
| **Composition** | Locked at publication; never changes | May be modified through formal reconstitution events |
| **Maturity** | Defined by resolution of all constituent markets | Open-ended; series runs until archived |
| **Market additions** | Not possible | Added through reconstitution |
| **Market removals** | Not possible | Rules-based only – maintenance criteria (two consecutive failed monthly reviews) or an extraordinary integrity event, per [Index Governance](/trust/benchmark-governance); never discretionary. Resolved markets exit via settlement |
| **Review cadence** | N/A | Committed at creation (default: monthly) |
| **Index history** | Continuous from inception to terminal value | Continuous across reconstitutions via chain-linking |
| **Publication pause during reconstitution** | N/A | Yes – during the reconstitution gate |
Belief Systems publishes both Fixed and Perpetual series. Perpetual is the right classification for thematic series whose underlying domain evolves over time – for example, indices tracking geopolitical conflict risk where new prediction markets emerge as new situations develop.
## The Analogy
A Perpetual series works like a reconstituting equity index. The S\&P 500 does not hold the same 500 companies forever – its index committee periodically adds and removes constituents as the market evolves, and the index level remains continuous across those reconstitutions.
A Perpetual Belief Index series follows the same convention:
* New constituent markets are added through a formal reconstitution event initiated by Belief Systems.
* The change is a calculation-level operation: the new composition and its weights are written, and the level is re-anchored to it. No trading takes place — Belief Index series are theoretical baskets, not portfolios.
* The reported index level is continuous across the event – no visible step in the chart.
Reconstitution is initiated by Belief Systems and subject to a published review cadence. It is
never retroactive – composition changes are forward-looking.
## Market Selection Criteria
Candidate markets for inclusion in a Perpetual series are evaluated across six dimensions. Most are enforced as hard numeric thresholds – the current values, and the weighting formula applied to admitted markets, are published in [Market Selection & Weighting](/indices/selection-and-weighting). The same rule set governs the series' initial composition, so a market that qualifies at creation would also qualify at reconstitution.
| Criterion | What it means |
| -------------------------- | --------------------------------------------------------------------------------------------------------------------- |
| **Thematic relevance** | The market must be directly related to the series' stated theme or risk domain. |
| **Liquidity depth** | The market should exhibit sufficient order book depth to support entry at reasonable size without excessive slippage. |
| **Trading volume** | The market should demonstrate meaningful daily trading activity, indicating sustained participant interest. |
| **Bid-ask spread quality** | The market should have reasonably tight spreads, indicating healthy two-sided participation. |
| **Time horizon** | The market's expected resolution date should fall within the series' intended lifecycle. |
| **Outcome clarity** | The market's resolution criteria must be unambiguous and verifiable. |
Individual series may apply additional theme-specific criteria on top of this baseline – those are disclosed on the series detail page when present.
## Review Cadence
Each Perpetual series specifies a review cadence at creation. The default cadence is **monthly**.
* The review cadence is a commitment to evaluate the series composition on that frequency – not a commitment to change it.
* A review also tests existing constituents against the maintenance criteria in [Index Governance](/trust/benchmark-governance); a constituent failing them at two consecutive reviews becomes a deletion candidate. A review may conclude that no new market meets the selection criteria, in which case the composition is unchanged and the outcome is recorded.
* The review cadence is displayed on each series' detail page.
Belief Systems commits to a consistent review rhythm rather than ad-hoc reconstitution. Reviews that result in no change are still reviews; the discipline is in the schedule, not in any given outcome.
## Chain-Linking (Index Continuity)
When composition changes, the raw probability-weighted aggregate (`raw_nav`) changes mechanically – different markets, different weights, different numbers. A Perpetual series uses **chain-linking** to ensure the reported index level is continuous across the change.
The index level immediately before reconstitution is recorded as the chain-link anchor.
A new `inception_raw_nav` is computed such that the first post-reconstitution index level equals
the anchor.
The index level continues from the anchor point under the new composition. Future movements
reflect the new basket, measured against the re-anchored inception value.
The arithmetic is a single Decimal expression evaluated to 8 decimal places. The chart is continuous; the composition change is reflected only in the underlying market breakdown.
`raw_nav` is a snapshot of the current composition's probability-weighted aggregate. When the
composition changes, `raw_nav` changes too – it has to, because the markets and weights are
different. `index_level` is the published time series; chain-linking re-anchors
`inception_raw_nav` at each reconstitution so that `index_level` stays continuous even as the
underlying basket evolves. The chain-link formula and a worked example are in [Index Calculation
Methodology – Chain-Linking](/indices/nav-methodology#step-5-chain-linking-perpetual-series).
## Replication Cost (Not Reflected in the Index)
A Belief Index level is a theoretical measurement. Anyone attempting to replicate a basket in the underlying markets would pay spreads, slippage, and order-placement costs, and would pay them again at each reconstitution. That cost is real, and it is **not** deducted from the published level – exactly as an equity index is published gross of the trading costs an index fund incurs tracking it.
* The published level is theoretical throughout: it never reflects an executed trade.
* Replication cost is disclosed as a methodology limitation, not netted into the number. See [Risk Factors](/risk/risk-factors).
## The Reconstitution Gate
During a reconstitution, the series enters a **gate** – a planned, bounded pause on publication while the composition change is written and verified.
| Dimension | Behavior during the gate |
| --------------------- | ------------------------------------------------------------------------------------------------------------------- |
| **Index computation** | Suspended. The last pre-gate index value remains authoritative, and no new level is published until the gate lifts. |
| **Other series** | Unaffected. The gate is scoped to the reconstituting series only. |
| **Typical duration** | Resolves within roughly 48 hours of activation. |
The reconstitution gate exists because, between the moment a composition change is activated and
the moment it is committed and verified, the series is in a mixed state. Publishing a level in
that window would produce an indeterminate value. The gate guarantees that no level is published
against a half-applied composition.
The gate lifts automatically on success – when the new composition is committed, the chain-link anchor is written, and verification passes. No manual acknowledgment is required to resume publication.
If the change cannot complete, the reconstitution follows an abort path: the old composition is restored and the gate lifts. Aborted reconstitutions are visible in the reconstitution history for the series.
## Audit Trail
Every reconstitution event produces a complete, auditable record:
* Date and time of the event.
* Markets added, with question text and condition identifiers.
* Weight changes: before and after, for each market.
* Chain-link anchor: the index level preserved across the event.
* Composition version: the before and after SHA256-tagged identifiers.
* Event status: completed or aborted, with reason.
The reconstitution history is visible on the series detail page. Aborted reconstitutions are visible alongside completed ones.
## What This Does Not Change
Everything else about how a Belief Index series works continues to hold for Perpetual series:
* **Observable inputs.** Every constituent price is a public order book midprice, and every resolution is confirmed against on-chain settlement state. The level is replicable from public data.
* **Valuation-window cadence.** Outside of the gate, Perpetual series are valued and published on the same valuation-window cadence as Fixed series.
* **Rules-based methodology.** Market selection for inclusion in a Perpetual series follows the published eligibility screens and weighting formula in [Market Selection & Weighting](/indices/selection-and-weighting).
* **Full audit trail.** Every reconstitution event produces a complete record – composition version from and to, chain-link anchor, and event outcome.
## Further Reading
The published eligibility thresholds and weighting formula applied at every reconstitution.
Including the chain-linking arithmetic and a worked example (Step 5).
Foundational concepts: composition, weighting, lifecycle.
Risks including concentration, correlation, resolution, and reconstitution.
Term definitions including chain-linking, reconstitution, and maturity type.