Fixed Income - Forward rates notation


it may be very obvious to some of you but I am struggling

Cannot get my head around this notation

f( n-1,1) or f(1,n-1) what do these mean. I am used to f(2,1) or f(1,1)

Huge thanks

n typically stands for the last period and would be typically be provided. n could be for instance 5 and then you just plug in.

f(1,2) means a 2 year loan starting in 1 year, f(time until loan starts, length of the loan)

So n-1 from is equal to j+k ?

0 -------- j ----------- j+k