Municipal Bond

When an American city builds a school, a water system or a toll road, it borrows in the municipal bond market — some $4 trillion of debt issued by states, cities, counties and thousands of local authorities.

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.

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 \(t\):

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.

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:

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.