Bond ETFs: The Basics

From one loan to a basket of thousands

A single bond has three defining terms: the issuer (a government or company), the coupon, and the maturity date. Its risks are two: the issuer failing to pay (credit risk) and interest rates changing (rate risk, explained below). A bond fund industrializes the idea exactly as equity funds did: an index defines a universe (say, euro-denominated investment-grade government and corporate bonds above a size threshold), the fund holds it, sampled where the market's thousands of illiquid line items make full replication silly (Chapter 11's spectrum applies with extra force here), and coupons flow out or accumulate per share class.

One structural difference from equities matters immediately: bond indices are weighted by debt outstanding. The biggest weights go to the biggest borrowers, which is not the same logic as rewarding the biggest earners. It's less damning than it sounds (big sovereign borrowers are usually the most liquid, highest-rated issuers), but it's a property to know you own.

Yield: what the basket actually earns

The number that describes a bond ETF's earning power is the yield to maturity (YTM), sometimes shown as "yield to worst": the annualized return the current portfolio would deliver if held to maturity at today's prices, coupons reinvested. It is forward-looking and comparable across funds, and it is not the same as the distribution yield (the trailing payouts, Chapter 10's warning applies squarely here: old coupons from a different rate era can make a fund's payout look richer or poorer than its true earning power).

The working rule: when comparing bond funds or judging what bonds offer you, read the YTM on the factsheet, dated, and largely ignore the distribution yield. As of any given day, a fund's YTM minus its TER is the honest baseline expectation for its long-run return, before rate changes bend the path.

Duration: the risk dial, in one intuition

Bond prices and interest rates sit on a see-saw. If you hold a bond paying 2% and the market's new bonds pay 4%, nobody buys yours at face value; its price falls until its yield matches. Duration measures the see-saw's leverage: a fund with duration 7 loses roughly 7% of its price when yields rise by one percentage point, and gains roughly 7% when they fall.

This single number explains most of what confuses bond-fund beginners:

• Why "safe" funds fell in 2022: yields rose ~3 points in a year; a duration-7 aggregate fund fell ~15%+, and long-duration government funds fell 25–40%. Nothing malfunctioned; the see-saw worked exactly as printed on the factsheet, on portfolios whose duration nobody had read.
• Why short-duration and money-market funds barely moved: duration near zero, see-saw leverage near zero.
• Why falling rates are a bond fund's party: the same arithmetic in reverse, which is what "bonds cushion equity crashes" historically meant, in the crashes where central banks cut rates.

There is a consolation built into the math: after a rate-driven price fall, the fund's YTM is higher, and an investor whose horizon exceeds the duration is roughly made whole by the better reinvestment. Duration is therefore also a matching tool: hold bond funds whose duration is comfortably shorter than your horizon for that money.

Credit: the other axis

The second risk dial is who you're lending to. The ratings ladder runs from AAA sovereigns through investment-grade corporates (BBB− and above) to high yield ("junk", below BBB−). Moving down the ladder buys extra yield and imports equity-like behavior: high-yield funds fall with stock markets in crises, precisely when you wanted the stabilizer to stabilize. The consequence for portfolio design is clean: the bond sleeve that exists to steady an equity-heavy portfolio should be high-grade and duration-appropriate; yield-reaching within it quietly converts the stabilizer into more equity risk wearing a bond costume. High yield can be a deliberate satellite; it cannot be the shock absorber.

The mainstream implementations, by role: aggregate funds (the "whole bond market" of a currency: governments + investment-grade credit, duration ~6–7) as the default single stabilizer; government-only funds for the purest crisis asset; short-duration/0–3yr funds and money-market ETFs for the calm end (and the instrument our modern-glidepath house view leans on); inflation-linked funds as the specialist tool for real-rate protection (own chapter later).

"But a fund never matures!" — the honest version

The classic objection: a single bond held to maturity returns its face value with certainty (default aside), while a fund rolls forever and its price can be down when you need it. The honest reconciliation: a bond fund behaves like a constantly renewed ladder of bonds; a ladder of individual bonds behaves like a fund. The certainty of one bond's maturity date is real and occasionally exactly what a known future liability needs (target-maturity bond ETFs exist for this, holding bonds to a single year and then dissolving). For the general stabilizer role, though, the fund's diversification across hundreds of issuers, at institutional trading costs, wins for almost everyone. The full mechanics, including how fund prices, YTM and horizon interlock, get Chapter 43; this chapter's rule of thumb suffices: duration shorter than horizon, credit quality matched to the job.

A worked example

Nadia, 45, holds 70/30 and is choosing her 30% bond sleeve (€60,000), horizon 15+ years.

• Fund A, aggregate EUR: YTM 3.1%, TER 0.10%, duration 6.5. Baseline expectation ≈ 3.0%/yr; a 1-point rate rise would dent it ~6.5% before the higher yield heals it over ~6–7 years, well inside her horizon.
• Fund B, EUR high yield: YTM 5.8%, duration 3.2. Tempting, until she checks 2008/2020 behavior: drawdowns of 15–35%, in sync with her equities. As a stabilizer it fails the job description regardless of yield.
• Fund C, 0–1yr governments: YTM 2.4%, duration 0.5. Maximum calm, lowest expected return; the right tool for money needed soon, oversized caution for a 15-year sleeve.

She takes Fund A for the sleeve, notes Fund C as where next year's planned kitchen renovation money lives, and files Fund B under "satellite, someday, sized like one." Three funds, three jobs, zero yield-chasing: the whole chapter in one decision.