Finance-Theory · Unit 5 · Video 2 · Interactive Practice
Why Bond Dealers Break: Thin Trading, One-Way Selling, and October 1987
IKey Formulas
Formula
Name
What it says
turnover=amount outstandingtraded per day
Daily turnover
Corporate bonds in Q2 2026: $66.9 billion a day against $12.1 trillion outstanding, so 12,10066.9≈0.55% of the market changes hands in a day
Iend=Istart+bought−sold
Dealer inventory
Bonds bought from sellers minus bonds sold to buyers: two-way traffic leaves I where it was, one-way selling adds to it round after round
profit=f⋅n+I⋅ΔP
Toll plus inventory
Over a round in which the price moves by ΔP: the toll f on the n bonds traded is earned whichever way they go, and the inventory term is the risk, safe from every price move only when I=0
own capitalloss=L⋅positionloss
Leverage on the book
With L=position/own capital: the borrowed money is owed in full, so every loss comes out of own capital, multiplied by L
Key Insight: A dealer's toll depends on how much traffic passes; its risk depends on which way the traffic runs. When the selling runs one way, the dealer's inventory grows while the price falls, and on leverage every markdown comes out of a thin layer of capital. On 19 October 1987 the NYSE specialists lost roughly a third of their combined capital doing exactly the job they were there to do.
IIEvery Minute, but Not the Same Bonds
Bonds trade somewhere every minute of the day, yet any one issue trades only now and then.
💡 Where the real volume sits: in the second quarter of 2026 U.S. fixed income traded about $1.6 trillion a day, $1,170.0 billion of it in Treasuries and $357.8 billion in agency MBS, while corporates traded $66.9 billion, municipals $14.6 billion and asset-backed securities $2.6 billion.
IIIThe Toll Collector and the One-Way Road
A dealer earns its toll on traffic in either direction, but bears risk whenever the traffic is one-sided.
IV19 October 1987: Buying All the Way Down
Buying as the price fell, on borrowed money, a specialist paid every markdown out of a thin layer of capital.
💡 In 2008 the same pattern played out in fixed income: while investors crowded into Treasuries, the heavy selling was in mortgage-related securities and riskier corporate bonds, which trade over the counter and far less often than stocks, and the dealers who normally stood in the middle were left holding them on leveraged books.
VQuiz Questions
Problem 1 · How Much of the Market Trades
Given: in the second quarter of 2026 the $12.1 trillion U.S. corporate bond market traded about $66.9 billion a day. What fraction of the corporate bonds outstanding changed hands on a typical day?
✅ Correct!12,10066.9≈0.0055: about half of one percent of a $12.1 trillion market changes hands on a typical day.
❌ Check the units. $12.1 trillion is 12,100 billion, not 1,210 billion, so dividing by 1,210 makes the answer ten times too large.
❌ That is the corporate share of the day's trading, not of the market.1,611.966.9≈4.2% compares corporate trading with the trading in all five sectors. The question divides by the amount outstanding.
❌ Ten times too small. $12.1 trillion is 12,100 billion, not 121,000 billion.
Show solution
Put both figures in the same unit: $12.1 trillion is $12,100 billion. Then
turnover=amount outstandingtraded per day=12,10066.9=0.00553≈0.55%a day
Turned around, 66.912,100≈181: at that pace it takes about 181 trading days for the volume to add up to what is outstanding.
The 4.2% answers a different question. The five sectors in the video trade total=1,170.0+357.8+66.9+14.6+2.6=1,611.9 ($ billions) a day, about $1.6 trillion, and corporates are 1,611.966.9≈4.2% of that. That is where the trading is, not how much of the corporate market trades.
Problem 2 · The Toll on a One-Way Afternoon
Given: a dealer collects a toll of $0.10 on every bond it trades. One afternoon 400 bonds arrive from sellers and no buyers come. The dealer buys all 400 at $100 each and collects its toll on each; by the close their price is $97. What is the dealer's profit for the afternoon, marking the bonds at the close?
✅ Correct! The toll was still collected, $40 of it, but the 400 bonds the dealer could not pass on lost $3 each: $1,200. Heavy traffic in one direction is a loss, not a good day.
❌ That counts only the toll. With no buyers the dealer is left holding all 400 bonds, and at the close each is worth $3 less. The inventory term I⋅ΔP=400×(−3) is part of the afternoon's profit.
❌ The toll is charged on trades that happened. Only 400 bonds changed hands: the dealer bought them and sold none, so the tolls are f⋅n=0.10×400=40, not 80.
❌ Close, but the toll still counts. The dealer collected $0.10 on each of the 400 bonds it bought, and that $40 offsets part of the $1,200 markdown.
Show solution
The afternoon's traffic is n=400 bonds, all in one direction, so the inventory rises from I=0 to I=400, and the price moves by ΔP=97−100=−3. In dollars,
profit=f⋅n+I⋅ΔP=0.10×400+400×(−3)=40−1,200=−1,160
Compare an afternoon of two-way traffic in which 400 bonds arrive from sellers and 400 go out to buyers. Then n=800 and I=0, so profit=0.10×800=80 ($80) whatever happens to the price: the toll collector's ideal, a fee on traffic running back and forth and no risk.
Problem 3 · A Stylised Specialist's Day
Given: a specialist faces one-way selling in its stock. When the stock opens it buys 10,000 shares at $50; as the selling continues it buys 10,000 more at $45 and another 10,000 at $40. The stock closes at $40, 20% below its opening price. Every purchase was paid 1 part from the firm's own capital to 2 parts borrowed.
What is the loss on the 30,000 shares at the close?
What fraction of the own capital put in is lost?
✅ Correct! The stock fell 20%, the position lost a ninth of what was paid, and leverage of 3 made that a third of the capital. The $900,000 borrowed is still owed in full.
❌ The 20% fall applies only to the first block. The specialist kept buying as the price fell, so the block bought at $45 is down $5 a share and the block bought at $40 is down nothing. Mark each block to the $40 close separately.
❌ Not every share was bought at $50. Only the first 10,000 shares cost $50; the later blocks were bought at $45 and $40, so they have less to lose.
❌ One block is missing. The shares bought at $50 lost $100,000, but the shares bought at $45 are also below cost, by $5 each.
❌ That is the loss as a fraction of the whole position. The lenders' $900,000 is owed in full, so the loss is charged to the $450,000 of own capital alone, which multiplies the fraction by L=3.
❌ The loss does not come out of the borrowed money. What is borrowed is owed in full; the $150,000 comes out of the $450,000 of own capital.
Show solution
Step 1: the loss, block by block. Mark each 10,000-share block to the $40 close:
The same number from cost and value: cost=10,000×(50+45+40)=1,350,000 and value=30,000×40=1,200,000, a loss of $150,000.
Step 2: the capital. One part in three of the $1,350,000 paid was the firm's own money:
own capital=31,350,000=450,000,borrowed=900,000
Step 3: the fraction lost.
own capitalloss=450,000150,000=31=L⋅positionloss=3×91
The stock fell 20%, but the position lost only a ninth of what was paid, because the later blocks were bought lower. Leverage of 3 turned that ninth into a third of the capital.
Problem 4 · When Holding Is Least Affordable
Given: suppose a bond dealer holds $200 million of riskier corporate bonds on $10 million of its own capital, the rest borrowed, and keeps its holdings at no more than 20 times its capital. One-way selling knocks 3% off the bonds' price. How much of the bonds must it sell to get back within its 20-to-1 limit?
✅ Correct! The $6 million loss leaves $4 million of capital, enough to carry only $80 million of bonds, so the dealer has to sell $114 million into the falling market.
❌ The loss came out of capital.10194=19.4 uses the capital from before the fall. The $6 million markdown leaves $4 million, and 4194=48.5 times capital is far over the limit.
❌ Selling bonds at the market price does not restore lost capital. It only swaps bonds for cash. The limit is on the size of the holdings: 20 × $4 million = $80 million of bonds at most.
❌ The bonds are no longer worth $200 million. After the 3% fall they are worth $194 million, so the sale needed is sale=194−80=114, in $ millions.
Show solution
Step 1: the loss, all of it out of capital. In $ millions,
bonds=200×0.97=194,loss=200−194=6,capital=10−6=4
With L=10200=20, the leverage rule gives the same thing: a 3% fall costs L×3%=20×3%=60% of the capital, which is $6 million of $10 million.
Step 2: the new limit.
limit=20×4=80,sale=194−80=114
Selling at the market price costs no further capital, so after the sale the dealer holds $80 million on $4 million, exactly 20 to 1.
A 3% fall cut the holdings the dealer can carry from $200 million to $80 million: the intermediary that stood in the middle has to sell into the same falling market. That is the video's closing point: how easily a bond can be sold depends on dealers being willing and able to hold it for a while, and a market where everyone is selling is exactly when they can least afford to.