where x2 is a single variable, and there is a single instrument, z1 that is relevant and exogenous, then the parameters of the model, (B, l), can be estimated using the moments of (y, x1, x2, z1). The IV estimator in (8-6) shows the one function of the moments that can be used for the estimation. In this case, (B, l) are said to be exactly identified. There are exactly enough moments for estimation of the parameters. If there were a second exogenous and relevant instrument, say z2 then we could use z2 instead of z1 in (8-6) and obtain a second, different estimator. In this case,