I am reading about t-distributions in a rather non-rigorous book.
An example is given here where the sample size is 25. The example also gives a numerical table used to determine the probability of getting a given numerical t-value $ t \geq x $ by chance.
But the example says that to determine this probability, we should look at the table row which says 24 degrees of freedom and not 25. Is that a typo or actually a rule i.e. when the sample size is 25 how many are the degrees of freedom? Also why are they named this way?