Meaning and comparability
A yield curve requires the currency, observation set, date and calculation method to be specified. FINRA discusses the relationship between maturities and yields and separately identifies the spread between maturities.[15](#source-cm-bonds)[19](#source-cm-spread) The yield of a coupon-bearing security, a spot rate and a calculated forward value are not automatically the same objects.
Limits of interpretation
A line through different credit qualities or instruments with different rights may mix several causes. The slope alone does not guarantee the future path of rates. A forward calculation requires consistent accrual rules; the resulting rate expresses a model relationship, not a promise of an available future placement. When comparing historical charts, check that the methodology has remained unchanged. The [yield-curve guide](/guides/yield-curve-spot-forward/) contains a fictional reproducible example and shows why a model value must not be declared a forecast.