Two identical instalments at opposite ends of a loan do very different work. The early ones are mostly interest and the later ones mostly principal, and the reason is arithmetic rather than policy.
Interest is charged on what is outstanding
Each period's interest is calculated on the balance remaining at that point. At the start of a loan that balance is at its maximum, so the interest charge is at its largest.
The instalment is fixed, so whatever is left after interest reduces the principal. Early on that residual is small.
As the balance falls, the interest portion falls with it and the principal portion grows. The instalment has not changed; its composition has.
The curve is steepest on long terms
The longer the term and the higher the rate, the more of each early payment interest consumes. On a long mortgage the first years reduce the balance remarkably little.
This is why the halfway point in time is nowhere near the halfway point in balance. Progress accelerates in the later stages.
Shortening the term raises the instalment but changes the composition immediately, because more of each payment must go to principal for the loan to finish sooner.
Overpayments act on the principal directly
An overpayment applied to the balance removes principal that would otherwise have accrued interest for every remaining period. Its effect is larger than its size suggests.
Because the saving comes from the periods that follow, the same overpayment made earlier saves more than one made later. Timing is the main variable.
Some agreements apply overpayments only at defined points or charge for them, so the terms determine whether the arithmetic works as expected.
Refinancing restarts the composition
Replacing a partly repaid loan with a new one of the original length returns the borrower to the beginning of the curve, where interest again dominates each payment.
A lower rate can still improve the outcome, but the comparison must be against the remaining term of the existing loan rather than against its original one.
Repeated refinancing to the full term is how a borrower can pay steadily for many years while the balance barely moves.
What the schedule actually tells you
An amortisation schedule shows the split for every instalment across the life of the loan. It is the clearest statement of what a borrowing decision costs.
Reading the total interest figure alongside the monthly payment reframes the choice, because the two respond to term in opposite directions.
Nothing about the structure is hidden, but it is rarely presented at the point of decision, where the monthly figure dominates the conversation.