Acceleration is the consequence that changes the borrower's arithmetic. Consumer credit agreements almost always contain an acceleration clause: on default the lender may declare the entire unpaid balance immediately due and payable. A borrower who has been thinking in monthly payments is now facing a single number, and no amount of catching up on the missed installments necessarily undoes it, because the lender's demand is for everything rather than for the arrears.
There is a precision here worth having, because it cuts the other way. On a loan where interest accrues on the balance actually outstanding, accelerating makes the principal plus interest accrued to that date due. It does not entitle the lender to the interest that had not yet accrued, because that interest was never owed. Where the loan is a precomputed transaction, meaning the interest for the full term was calculated in advance and written into the note, the unearned portion has to come back: 15 USC 1615(a)(1) requires the creditor to refund promptly any unearned portion of the interest charge, and (a)(3) applies that right "without regard to the manner or the reason for the prepayment," naming among its examples "any prepayment made as a result of the acceleration of the obligation to repay the amount due with respect to the transaction." So acceleration is the case the provision was written to cover, not an argument by analogy from it.
Default is often not about money at all. A credit agreement can make any number of things a condition. Common non-monetary triggers include failing to maintain the insurance the lender requires on the collateral, selling or transferring the secured property without consent under a due-on-sale clause, moving titled collateral out of state, giving materially false information on the application, and a cross-default provision under which defaulting on one obligation to the lender puts the others in default too. None of these involves a missed payment, and a borrower can be paying perfectly and still be in default.
One of those triggers is defined in federal law, which is a useful check on how real they are. 12 USC 1701j-3(a)(1) describes a due-on-sale clause as a contract provision authorizing a lender, at its option, to declare the secured sums due and payable if the property securing the loan "is sold or transferred without the lender's prior written consent," and subsection (b) preempts state law that would prohibit enforcement. It is not unlimited: on residential property of fewer than five units, subsection (d) bars the lender from exercising the option on a list of transfers that includes one to a spouse or children, one arising from a divorce decree, one on the death of a borrower or a joint tenant, and a transfer into an inter vivos trust the borrower remains a beneficiary of.
How far such a clause can reach is easiest to see from a place Congress had to step in. Private student loan agreements once commonly allowed a lender to declare default or accelerate against the student borrower because a cosigner had died or filed bankruptcy. Since a 2018 amendment, 15 USC 1650(g)(1) prohibits exactly that. The prohibition exists because the practice did, and it is a narrow fix to one product rather than a general rule about automatic-default clauses.
What follows a default is set by whether the debt is secured, and the two paths barely resemble each other. A secured lender already holds a lien on identified property, so on default it can pursue the collateral: repossession of personal property, foreclosure of real property, each with its own state-law notice and cure requirements. An unsecured creditor holds nothing but a promise, so its route runs through a lawsuit, a judgment, and only then the post-judgment tools. The secured debt and unsecured debt pages carry those two paths, and the practical point for a borrower in trouble is that the type of debt, not the size of it, decides how fast and how far a default can reach.
One common consumer loan sets its own day count by regulation rather than by contract, and it is the exception that proves the rule. For a federal Direct Loan, default is defined administratively rather than left to the promissory note, and it arrives after a long fixed period of non-payment with prescribed consequences attached, including collection powers no private lender has. That regime, and the cures available inside it, belong to the student loan default page and the Student Loans guide. A 401(k) loan is another special case that looks like this one and is not: an unpaid plan loan becomes a deemed distribution or a plan loan offset, which are tax events rather than acceleration, and the 401(k) loan page keeps that distinction.