rt class was used for each
   different assertion. We call this the 
   <a href="classicModel.html">classic model</a>. 
   It continues to be supported in NUnit, since many people prefer it.</p>

<p>Beginning with NUnit 2.4, a new 
   <a href="constraintModel.html">constraint-based model</a>   was introduced. This approach uses a single method of the Assert class
   for all assertions, passing a <b>Constraint</b> object that specifies 
   the test to be performed.

<p>This constraint-based model is now used