“Fixed rate” does not always mean “the same cash payment.” US Treasury Inflation-Protected Securities provide a useful example because their interest calculation uses an adjusted principal.
Keep rate, principal and payment on separate lines
TreasuryDirect’s TIPS explanation states that principal adjusts with inflation or deflation and interest is paid every six months at a fixed rate. The interest amount therefore varies with the principal used in the calculation.
For a simplified hypothetical example, take a 2% annual rate and an adjusted principal of $1,020. Half a year’s interest is $1,020 × 0.02 ÷ 2, or $10.20. If the relevant adjusted principal were $1,000 instead, that calculation would produce $10. The rate is unchanged; the base is different.
Do not turn the example into a return forecast
This arithmetic leaves out purchase price, timing conventions, taxes and any sale before maturity. It does not tell you the return on a particular investment. TreasuryDirect also explains the principal protection that applies at maturity; that is a separate condition from the market price at which a holding might be sold earlier.
When reading a money book, mark which quantity each sentence describes. Then follow the book’s example through one payment before extending it across years. If the text switches between coupon, yield and inflation without identifying the base of each calculation, pause and consult the security’s official terms. This article is an explanation of terminology, not a recommendation to buy a bond.
AI-generated editorial illustration; not a documentary photograph.