3 is a landmark decision in Canadian constitutional law by the Supreme Court of Canada (SCC) relating to the division of powers between Federal and Provincial legislative bodies.

There is an inquiry into the true nature of the law in question for the purpose of identifying the matter to which it essentially relates.

[4] Also, some matters are by their very nature impossible to categorize under a single head of power: they may have both provincial and federal aspects.

The double aspect doctrine, which applies in the course of a pith and substance analysis, ensures that the policies of the elected legislators of both levels of government are respected.

In practice, it is largely reserved for heads of power that deal with federal things, persons, or undertakings, or if, in the past, its application has been considered absolutely indispensable or necessary to achieve the purpose for which exclusive legislative jurisdiction was conferred, as discerned from the constitutional division of powers as a whole, or what is absolutely indispensable or necessary to enable an undertaking to carry out its mandate in what makes it specifically of one jurisdiction or the other.

It is only if the adverse impact of a law adopted by one level of government increases in severity from affecting to impairing that the core competence of the other level of government or the vital or essential part of an undertaking it duly constitutes is placed in jeopardy.