Municipal Bond
Also known as: Muni, Tax-exempt bond, GO bond, Revenue bond
Lending to cities, states and school districts — with the US tax code, not the coupon, doing half the work.
1 · SnapshotThe one idea to remember
2 · BeginnerWhat is it, really?
When an American city builds a school, a water system or a toll road, it borrows in the municipal bond market — debt issued by states, cities, counties and thousands of separate local authorities, each with its own credit.
The defining feature isn't the borrower — it's the tax break. Interest on most munis is exempt from US federal income tax, and usually from state tax for in-state residents. A muni paying 3% can therefore be a better deal for a high-earning investor than a corporate bond paying 4.5%: what matters is what survives taxation, not the headline number.
Munis come in two basic kinds. A general obligation (GO) bond is backed by the issuer's power to tax — the full faith and credit of the city. A revenue bond is backed only by the cash from a specific project: the tolls of the bridge, the fees of the airport. If the project disappoints, the taxpayer owes you nothing.
Point at a line to read what it is doing.
How do I read this chart?
Yield runs across, price up. The line falls, which is the whole of the relationship between the two — and it is curved rather than straight, which is convexity and the reason a bond gains more when yields fall than it loses when they rise by the same amount.
Where this is explained properly:
The axes carry no scale on purpose. Every line here is a stylised shape drawn to show a mechanism, so there is no number to read off one — and any figure taken from it would be invented.
- Asset class
- Fixed income (sub-sovereign)
- Instrument type
- Bond — general obligation or revenue
- Traded
- OTC dealer market, thousands of small issuers
- Typical users
- US taxable individuals, insurers, muni funds
Which risks decide the outcome
Not how risky this is, and not a rating — there is deliberately no total. It says which of five failure modes drives what happens here, in the same order on all 129 products so they can be compared. This publication's own reading; see the notice below.
- Marketmatters
- Creditdecides it
- Liquiditydecides it
- Fundingbarely applies
- Operationalbarely applies
What decides it here. A public issuer's revenue decides whether it pays; the market's thinness decides what a sale costs you when you need one.
3 · IntermediateHow it works in practice
The core arithmetic: tax-equivalent yield
To compare a tax-exempt muni with a taxable bond, gross up by the investor's marginal tax rate \(t\):
What the symbols mean
- ythe yield to maturity
- ta point in time
A 3.0% muni for an investor at \(t = 40.8\%\) (37% top federal rate + 3.8% net investment income tax) is worth \(3.0\%/0.592 \approx 5.07\%\) taxable. The market-wide ratio \(y_{\text{muni}}/y_{\text{Treasury}}\) — the muni/Treasury ratio, historically 65–90% — is the market's implied clearing tax rate, and its swings are the muni market's main valuation signal.
Credit: better than its reputation
- Moody's cumulative 10-year default rates: investment-grade munis ≈ 0.1% versus ≈ 2% for same-rated corporates — the taxing power and essentiality of services make munis structurally safer, which the post-2008 recalibration of rating scales finally acknowledged.
- The famous failures — Detroit 2013 (~$18bn), Puerto Rico 2017 (~$70bn) — were concentrated, political, and slow-moving; recoveries varied wildly by lien and bond type.
- Bond insurance (Ambac, MBIA) once wrapped half the market to AAA; the monolines' 2008 collapse ended that era, and today most munis trade on their own credit.
Market structure quirks
- Retail-dominated and fragmented: ~50,000 issuers and a million CUSIPs; odd-lot retail trades pay spreads that institutional blocks don't.
- Serial issuance: one bond deal is sliced into maturities from 1 to 30 years — each slice tiny and illiquid.
- 10-year par calls are standard: most long munis are priced to a call, not to maturity.
4 · AdvancedPricing & valuation
The de minimis cliff
Market discount on munis is taxed as ordinary income unless it stays under the de minimis threshold — 0.25 points per complete year to maturity below par:
What the symbols mean
- Pa price, or a present value
- nhow many periods, or how many things
A bond through that floor sees its after-tax yield jump discontinuously worse, so in rising-rate markets discount munis gap down in price as they approach the cliff — a tax rule that manufactures negative convexity out of thin air. Dealers quote "kicker" bonds (high-coupon, priced to call) partly to keep retail paper safely above the threshold.
Yield curve and the ratio trade
The muni curve is persistently steeper than the Treasury curve: banks and insurers (taxable-yield buyers) anchor the front end, while the long end must clear through households alone — the "preferred habitat" story in its purest form. Crossover traders monetise dislocations in the ratio \(y_M/y_T\): buying 30y munis at ratios above ~100% (as in March 2020, when the ratio briefly exceeded 300%) is the classic distressed-liquidity trade, hedged with Treasury futures and financed patiently, since muni repo is thin.
Structural niches
- Taxable munis & BABs: pension deals and 2009–10 Build America Bonds pay taxable coupons with a federal subsidy — they trade like long corporates and attract global buyers the tax-exempt market can't reach.
- AMT paper: private-activity bonds (airports, housing) are taxable under the alternative minimum tax; the "AMT spread" of 10–30bp prices a minority tax regime.
- Pre-refunded munis: escrowed-to-Treasuries after an advance refunding — muni tax treatment on Treasury credit, the market's cleanest asset (the 2017 tax act ended new advance refundings, making the stock a melting ice cube).
- Tender option bonds (TOBs): leverage vehicles that strip long munis into floaters + inverse floaters; their unwind amplified the 2008 and 2020 muni sell-offs.
The formulas above are standard textbook formulations, simplified for teaching. They explain the mechanism — they are not a valuation tool, and they will not reproduce a dealer’s price.
5 · Desk notesHow practitioners think about it
Now say it back
Close the page and give Municipal Bond in four sentences. It takes a minute and it is the only way to find out whether reading it was enough.
- Who wants what — two parties wanted opposite things badly enough to write it down.
- What the contract obliges, and when — not the payoff; the obligation.
- Where the money comes from — name the source, or you have described a hope.
- What makes it lose — the ordinary way, not the dramatic one.