If you have ever looked at a brokerage account and seen a five-letter ticker ending in XX — SPAXX, VMFXX, FGXXX, SWVXX — you were looking at a money market fund. That specific ticker shape is a signalling convention: single-X endings mark ordinary open-end mutual funds (VTSAX, FXAIX), and the double-XX marks the money-market subclass. DiviDrip uses that shape as its primary classifier — every 3-letter- prefix + XX ticker is auto-flagged, and the app swaps in an MMF- specific Insights card, hides the Dividend Triangle, and hides the DDM Valuation Matrix on those pages.

A quick history

Money market funds were invented in 1971 by Bruce Bent and Henry B. R. Brown, who launched The Reserve Fund to route ordinary savers around Regulation Q — the Federal Reserve rule that capped the interest rate banks were allowed to pay on deposits. By pooling investor cash and buying short-term corporate and government debt, they could pass the higher market yields directly through to individual investors. The idea took off during the high-inflation 1970s and by the 1980s an MMF was the default cash sleeve inside every major brokerage.

The next big moment was September 16, 2008. The Reserve Primary Fund — a direct descendant of Bent and Brown's original fund — held commercial paper issued by Lehman Brothers. When Lehman filed for bankruptcy the day before, the Reserve Primary's NAV printed at $0.97, breaking the buck for the first time in decades. That triggered a run on the broader money market industry and prompted the U.S. Treasury to temporarily guarantee the entire sector. The event drove two waves of SEC reform (2014 and 2023): institutional prime funds now use a floating NAV and can impose liquidity fees or redemption gates; retail government and Treasury funds kept the stable $1.00 NAV.

How the $1.00 NAV works

The defining feature of a retail MMF is the constant $1.00 share price. The mechanism has two moving parts:

1. Amortized-cost accounting. SEC Rule 2a-7 allows MMFs to value their holdings at purchase price plus smoothly-accrued interest, rather than fluctuating live market price. Because the holdings are short-duration (portfolio maturity typically 30–60 days), the gap between amortized cost and true mark-to-market stays negligible under normal conditions.
2. Daily interest sweep. Whatever interest the underlying bills, repos, and commercial paper throw off during a given day is booked as a dividend receivable, not as an increase in NAV. Instead of the share price drifting from $1.00 to $1.001 to $1.002, the price resets to exactly $1.00 every close and the accrued interest queues up to be paid out at month-end.

You can think of the NAV like a water level held perfectly flat by draining every drop of new interest into a bucket on the side. The bucket is your dividend; the water level never moves.

What's inside an MMF

MMFs are legally restricted to short-duration, high-credit-quality debt. The exact mix depends on the fund category:

• U.S. Treasury bills — direct short-dated obligations of the U.S. Treasury. The gold standard for safety.
• Repurchase agreements (“repos”) — overnight loans collateralized by Treasuries or agency paper.
• Agency paper — short-duration debt from Federal Home Loan Banks and similar government-sponsored entities.
• Commercial paper — short-term unsecured notes from investment-grade corporations. Only prime MMFs hold this.
• Negotiable CDs — large-denomination bank certificates of deposit maturing in under 397 days.
• Municipal notes — short-term paper from state and local governments. Only muni MMFs hold this.

The four categories

The tax treatment and the risk profile both depend on what a specific MMF holds. In practice retail investors are picking between four categories:

• Government MMFs — hold at least 99.5% Treasuries, agency paper, and repos backed by those. Examples: SPAXX, VMFXX, FGXXX, SNVXX. The safest option. Distributions are federally taxable at ordinary rates; state and local tax treatment depends on the fraction attributable to direct U.S. Treasury obligations.
• Treasury MMFs — hold ≈100% direct U.S. Treasuries. Examples: VUSXX, SNSXX. Distributions attributable to direct Treasuries are exempt from state and local income tax — which makes them meaningfully better than government MMFs for residents of high-tax states like California, New York, or New Jersey.
• Prime MMFs — hold a mix of government paper and investment-grade commercial paper / CDs. Examples: SWVXX, SPRXX. Slightly higher yield in exchange for a sliver of extra credit risk (and the small chance of a liquidity fee or gate during severe market stress under post-2023 SEC rules).
• Municipal MMFs — hold short-duration muni paper. Examples: VMSXX, SWTXX. Distributions are federally tax-exempt; single-state muni MMFs (like SWKXX or VCTXX for California residents) are also state-exempt. Yields quote lower than taxable MMFs but the tax-equivalent yield can beat them for high-income investors in a standard taxable brokerage account — the yields page has a tax-equivalent calculator for exactly that comparison.

Yields, dividends, and the tax hit

The public yield for an MMF is the 7-day SEC yield: the past week's daily accruals annualized and net of expense ratio. Every US money fund reports it to the SEC monthly on Form N-MFP, share class by share class, and that filing is what DiviDrip reads — a market data vendor is only the fallback. It shows on the MMF Overview card alongside total net assets, and the money market fund yields page ranks every fund we track on that same measure, split by the four categories above.

Distributions are paid monthly on the last business day. They land as cash in your brokerage sweep or reinvest into fractional shares if you have DRIP turned on for the ticker. Either way, the tax treatment is the same and the tax treatment is the biggest gotcha for beginners:

• MMF dividends are ordinary (non-qualified) income — taxed at your marginal federal rate (10–37% in 2026). Not the friendlier 0/15/20% qualified-dividend rate that applies to most common-stock dividends.
• Even though the broker reports them on Form 1099-DIV (Box 1a, Ordinary Dividends), the IRS treats the payouts as interest — because the underlying is short-term debt, not corporate equity.
• Reinvestment does not defer the tax. If DRIP is on and you never touch the cash, you still owe ordinary income tax on every distribution in the year it's paid. That's the “reinvestment tax trap” — you can wind up owing tax on money you have to raise from other sources.
• State-tax carve-outs. Treasury MMFs are state-tax- exempt on the direct-Treasury portion. Municipal MMFs are federally tax-exempt. Single-state muni MMFs are state-exempt for residents of that state.

The expense ratio is the whole ballgame

Money funds all buy the same short-dated paper, so what separates their yields is mostly the fee. Across the funds we track, the ones charging 0.20% or less pay a median 7-day yield roughly 60–70 basis points higher than the ones charging 0.50% or more — on $50,000 of cash that is a few hundred dollars a year for holding a different ticker at the same broker. Two things follow: buy the cheapest share class you can actually access, and don't subtract the expense ratio from the quoted yield. The 7-day SEC yield is already net of fees, so the fee explains the yield rather than reducing it a second time. We read each fund's expense ratio from its SEC N-CSR shareholder report and show it beside the yield on the yields page.

Money market ETFs are a different product

A handful of exchange-traded money market funds now exist — GMMF, PMMF, JMMF, MMKT, SBIL. They are not 5-letter XX tickers and DiviDrip does not classify them as MMFs, correctly: they trade intraday at a floating market price near $100 a share, no broker sweeps cash into them, and they quote a trailing distribution yield rather than a 7-day SEC yield. They appear in their own section of the yields page, deliberately apart from the $1.00-NAV funds, because ranking the two yield types together compares a backward-looking number with a forward-looking one. You can see them, and how their distribution yields currently compare, on the money market fund yields page.

Why a Roth IRA is the tidiest wrapper

The ordinary-income tax hit disappears entirely inside a Roth. Every monthly distribution compounds tax-free, and qualified withdrawals (age 59½ plus the 5-year rule) come out tax-free. Two use cases where an MMF-inside-a-Roth pattern shows up a lot:

• Dry powder. Contribute your annual $7,500 (or $8,600 if you're 50+) at the start of the year, park it in a government MMF, and deploy into stocks when the market gives you an entry point you like. You collect the prevailing short-rate yield in the meantime — see the current 7-day yields.
• Backup emergency sleeve. Roth contributions can be withdrawn tax- and penalty-free any time. Keeping them in a $1.00 NAV MMF means the balance is always there at face value if you ever need to pull it out.

The main tradeoff: an MMF paying the short rate is unlikely to beat inflation over long horizons, and a Roth IRA's biggest advantage is decades of tax-free compound growth on higher-return assets. Using the whole account for cash wastes that advantage. Most professionals treat an MMF sleeve as a <20% allocation, not the whole thing.

Examples you'll actually see in a brokerage account

The following are all DiviDrip-classified MMFs (5-letter, XX suffix, confirmed quoteType: MONEYMARKET):

• SPAXX (Fidelity, Government MMF) — Default cash-sweep in Fidelity brokerage accounts.
• VMFXX (Vanguard, Government MMF) — Default settlement fund in Vanguard brokerage accounts.
• VUSXX (Vanguard, Treasury MMF) — ≈100% direct Treasuries — best for high-tax states (CA, NY, NJ) chasing state-tax exemption on cash yield.
• SWVXX (Schwab, Prime MMF) — Schwab’s retail prime option — holds commercial paper + CDs, accepts a sliver of extra credit risk for a little more yield.
• FGXXX (First American, Government MMF) — Institutional Class X — often used as underlying cash-sweep by advisors and thematic ETFs.
• VMSXX (Vanguard, Municipal MMF) — Federally tax-exempt — best for high-income investors in a taxable account.

How DiviDrip classifies and displays them

On every 5-letter XX ticker, DiviDrip:

• Stamps is_money_market_fund: true on the cache doc, plus mmf_family (Fidelity / Schwab / Vanguard / etc.), the live mmf_7day_yield, and total mmf_aum.
• Renders a Money Market Fund Overview card at the top of the Insights tab, showing the 7-day yield and total net assets as headline metric tiles.
• Hides the Dividend Triangle tab — Revenue × EPS × Dividend Growth doesn't apply to a constant-NAV product.
• Replaces the DDM Valuation Matrix and Target Buy Price with a small “N/A — Money Market Fund” note.
• Serves a dedicated MMF SEO section on the public stock page so search users landing on FGXXX or SPAXX see the $1.00-NAV explainer + risk note before the standard dividend body.
• Excludes MMFs from the Div → Non-Div admin reconcile panel — MMFs use mmf_7day_yield on a separate field, not dividend_yield, so they don't belong in that reconciliation queue.

The bottom line

Money market funds are the app's designated cash tier — useful for parking money you can't afford to lose to volatility, but not a wealth-building engine. Treat them like your brokerage's version of a checking account with a yield: safe, liquid, and earning short-rate interest, but never expected to appreciate. And keep them out of a standard taxable account when possible — the ordinary-income tax hit is the single biggest thing separating an MMF from an equally-yielding tax-favored alternative.

Not investment, tax, or legal advice. MMFs aim to maintain a constant $1.00 NAV but are not guaranteed to do so and are not FDIC-insured. Read the individual fund's prospectus before treating it as a cash-equivalent. Tax rules cited reflect 2026 U.S. federal law and can change; consult a tax professional for your specific situation.