Common law is comprised of the guidelines that are set forth from case law rather than actual legislature. Such case law represents all previous decisions made by judges according to the cases that came before them, which then act as a basis for judicial interpretation.
Such interpretation constitutes how exactly courts provide their interpretation of the law as we see by way of the constitution as well as other areas of legislation.
Despite the existence of statutes, we usually put most of the responsibility upon our judicial system in terms of such interpretation due to the fact that we may have varying opinions on such statutes. Such an example would be that of a child custody suit.
As the parties involved may be aware of, statutes exist stating that custody should be provided or conveyed with the "best interests" of the minor at the forefront of all considerations. This statement, though well and good, may be interpreted differently depending upon which side you listen to, however.
The mother, for instance, may believe that these "best interests" exist with her as the sole guardian as opposed to only a part time visitor. The specific way in which the judge may go about attaining case law, which will then lead to such a judicial interpretation is as follows. Judges will first seek out the law appropriate to the case at hand.
Afterward, they will usually compose some sort of document or form, which contains their own summary regarding their logic as concerning the law, also known as their legal "opinion." Such "judicial opinions" are what create the foundation for "common law."
This, then, is how the judge decides the law must operate in connection to the case they are currently presiding over. In such a circumstance, the judge must also look to precedence in terms of the rulings of judges that presided over similar cases before them in order to provide adequate judicial interpretations.
There does exist, however, a "hierarchy" of judges, so that not every judge is strictly subject to the precedence of another.
In terms of judicial interpretation of the constitution, there are a number of preordained approaches that must be followed. These include the "contextualist," "developmentalist," "doctrinalist," "originalist," "structuralist," and the "textualist/strict constructionist."
The contextualist approach deals mostly with the original conception and point of the text provided, such as why it resides where it does in the document and the history behind its advent as well as the intentions as well.
Developmentalists focus upon the step-by-step creation that has gone into a judicial document such as those of its evolution. A doctrinalist looks to precedence, such as that which is explained in terms of judge's judicial interpretation of case law.
Originalists, however, attempt to embody the mentalities of the "founding fathers," as they look to comprehend their intentions and methods of understanding. The structuralist approach look to the management of the authority of offices within the constitution.
The last approach used in judicial interpretation of the constitution is that of the textualist/strict constructionist approach, where the interpretation focuses upon the intended "literal meaning" behind such texts.