Identifier: C2T3L4 | Revision: 2026-10-26

Decision and objectives

An inflation-linked bond can protect a defined index exposure while failing to match a household's exact expenses. Identify the reference index, lag, base and floor before treating its name as a promise of purchasing-power preservation. Index-linked coupon and redemption mechanics are contractual; protection at final redemption does not imply a stable market price before that date.

Objectives: Calculate an index ratio and adjusted cash; Distinguish real yield from nominal return; Explain lag and spending-basket mismatch.

Prerequisites: C2T3L3.

Mechanics

A common structure multiplies a base principal and real coupon by an index ratio. Reference index means the particular published index used by the issue, sometimes interpolated and lagged. An indexation lag uses earlier inflation observations because publication takes time. A contract may protect original principal at maturity but not on sale; other structures have different floors. These features must be read, not assumed.

Price and return drivers

Index ratio = contractual reference index / base index. Adjusted principal = nominal base principal × ratio. A coupon based on an annual real rate r and m equal periods can be adjusted principal × r / m, subject to the issue's exact method. Real yield is the discount yield on inflation-adjusted purchasing-power cash under the relevant index convention. It is not the coupon, nor an inflation forecast.

A nominal model return and inflation can be related by real return = (1 + nominal return) / (1 + matching inflation) − 1. Subtracting percentages is only an approximation. Market real yields can rise and reduce today's value even while indexed principal grows.

Worked case

Hypothetical unlevered issue: base nominal TRY 100,000; base index 200; relevant lagged reference index 230; real annual coupon 4%, paid semiannually; assume both coupon and principal use the same ratio and no floor binds. Ratio = 230 / 200 = 1.15. Adjusted principal = TRY 115,000. Half-year coupon = 115,000 × 0.04 / 2 = TRY 2,300. If this is final maturity and principal is redeemed at adjusted par, cash = 115,000 + 2,300 = TRY 117,300 before costs and tax.

Suppose a separate one-year holding produced nominal return 18%, while matching inflation was 15%. Real return = 1.18 / 1.15 − 1 = 2.6087%. Calling that result a 3% real return would overstate the precise compounded outcome.

Adverse case

If the household's costs rise 25% while the lagged reference rises 15%, a principal claim of TRY 115,000 buys only 115,000 / 1.25 = TRY 92,000 of the original household basket: an 8% shortfall relative to initial TRY 100,000 purchasing power. This is basis mismatch, not necessarily a breach of the bond contract. A rise in required real yield can additionally depress the sale price; a positive index ratio does not guarantee a positive early-exit return.

Practical use

Retrieve the issue's base index, reference index publication, lag and interpolation rule, coupon basis and redemption floor. For domestic exchange quotes, verify whether price is stated excluding the inflation coefficient and how settlement applies it; do not multiply an already indexed dirty cash amount again. Borsa's procedure distinguishes inflation-adjusted settlement price. Investor tax may treat income components differently, so retain separate accrual, indexation and sale records rather than assume all inflation uplift is tax-free.

Decision boundaries

Use the instrument when its reference index and maturity reasonably fit the liability and the investor can tolerate real-yield volatility. It cannot exactly hedge every household basket or a foreign-currency liability. Decline a short-horizon trade justified only by a high recent inflation print: lagged indexation may already be priced and the exit yield can move adversely.

Practice and references

Exercise: hypothetical base face TRY 60,000, base index 300, reference index 336, real annual coupon 3%, annual payment. Answer: ratio 1.12; adjusted principal TRY 67,200; coupon TRY 2,016; final cash TRY 69,216 if adjusted par is redeemed with that coupon. No costs, floor effect or tax is included.

Takeaway: an index link hedges a specified calculation, while market real yields and personal inflation remain separate risks.

Reference guide: inflation-linked-bond, inflation-indexed-cash, Indexation lag, Real yield.

Next: C2T3L5. Topic test: C2T3.

Primary sources (checked 2026-10-26; changing terms require a publication recheck): Borsa Istanbul announcement amending Debt Securities Market Procedure — 9.1.5; Annex 1; quotation and settlement price distinctions; HMB Public Debt Management Report 2024 — Government Debt Securities; lease certificate definitions; borrowing instruments.

Shared reference guide