  We provide a detailed description of the model Hilbert space $L^2(\bbR;
d\Sigma; \cK)$, were $\cK$ represents a complex, separable Hilbert space, and
$\Sigma$ denotes a bounded operator-valued measure. In particular, we show that
several alternative approaches to such a construction in the literature are
equivalent.
  These spaces are of fundamental importance in the context of perturbation
theory of self-adjoint extensions of symmetric operators, and the spectral
theory of ordinary differential operators with operator-valued coefficients.
