Illustration of the knot placement for a spline of order M (e.g., M = 4 for a cubic spline) with N degrees of freedom.  [t_beg,t_end ]  is the spline interval over which the spline is fitted to given data points. The end knots τ_M − 1 and τ_N are at the endpoints of the spline interval. The N − M interior knots are chosen to give the spline the desired flexibility, including multiple knots where required. The placement of the anterior (τ ≤ t_beg) and posterior knots (τ ≥ t_end) is in principle arbitrary: it does not change the fitted spline in  [ t_beg,t_end ] , but may affect the condition number of the least-squares fit. The parameters of the fitted spline are the coefficients c₀,   ...   c_N − 1 of the B-splines B₀(t) through B_N − 1(t).