For B-Spline why $n+1 > k ge 2$ and why is $t_{k-1} le t le t_{n+1}$

For B-Spline why $n+1 > k ge 2$ and why is $t_{k-1} le t le t_{n+1}$

The general definition of B-Spline is

$$P(t)=sum_{i=0,n}N_{i,k}(t)P_i, t_{k-1} le t le t_{n+1}$$

Why can’t k (the degree of the basis functions) be greater then the number of control points(n+1)? And must be at least 2?

I swear I’ve seen b-splines on an interval of [0,1] where all the functions had degree 2 so that means $t_{k-1}=t_{1}$. But I also don’t really know how to interpret $t_i, iin[k-1,n+1]$

I read both here
https://www.ibiblio.org/e-notes/Splines/basis.html