Only legal nerd enjoy reading Supreme Court decisions, but here's a quick breakdown of how to read this (you don't have to read the whole thing to get it).
The first paragraph, reproduced below, is the 'summary,' which tells you what the case is about:
The telephone company, at police request, installed at its central offices a pen register to record the numbers dialed from the telephone at petitioner's home. Prior to his robbery trial, petitioner moved to suppress "all fruits derived from" the pen register. The Maryland trial court denied this motion, holding that the warrantless installation of the pen register did not violate the Fourth Amendment. Petitioner was convicted, and the Maryland Court of Appeals affirmed.
Next, the part after the word HELD: is known as the holding, and this is the part of the ruling which has legal force and constitutes precedent designed to guide courts going forward into the future.
Finally, the opinion (beginning with 'Mr. Justice Blackmun delivered the opinion of the court:') is an explanation of what the holding means by one of the judges who was in the majority (which particular judge is assigned at the whim of the Chief Justice, if s/he is with the majority, or otherwise the most senior Justice in the majority group). A concurrence is a supplementary or orthogonal explanation from another justice who supports the decision but for a slightly different legal reason. A dissent is an explanation of how a justice on the losing side thinks the law should have been interpreted, or addressing some perceived flaw in the opinion.
The opinion, concurrences, and dissent are (of course) of great interest to lawyers and judges who have to consider similar cases in the future, but it's important to remember that they're not law (lawyers collectively refer to them as dicta, meaning 'speeches' or 'sayings'.). Only the holding is law. I have seen decisions where the holding set precedent and the opinion is so logically or factually flawed as not to make sense, but bear in mind that you can't overturn a court's decision solely by pointing out flaws in the opinion, because the opinion itself is not law. You have to either point a contradiction or falsity within the holding itself (rare) or start over with reference to external sources (Constitution, legislation, prior precedent) to show why the holding is wrong.
It's very important, and often difficult, to distinguish between the holding and the dicta. I mention this because many people, and especially hackers, are liable to get caught caught in some ambiguity or linguistic argument about the contents of an opinion and mistakenly think that they've thereby invalidated the holding. Lawyers and even courts are not immune to this kind of mistake, and cases sometimes come apart because one side has relied too heavily on a particular sentence in an opinion.
The first paragraph, reproduced below, is the 'summary,' which tells you what the case is about:
The telephone company, at police request, installed at its central offices a pen register to record the numbers dialed from the telephone at petitioner's home. Prior to his robbery trial, petitioner moved to suppress "all fruits derived from" the pen register. The Maryland trial court denied this motion, holding that the warrantless installation of the pen register did not violate the Fourth Amendment. Petitioner was convicted, and the Maryland Court of Appeals affirmed.
Next, the part after the word HELD: is known as the holding, and this is the part of the ruling which has legal force and constitutes precedent designed to guide courts going forward into the future.
Finally, the opinion (beginning with 'Mr. Justice Blackmun delivered the opinion of the court:') is an explanation of what the holding means by one of the judges who was in the majority (which particular judge is assigned at the whim of the Chief Justice, if s/he is with the majority, or otherwise the most senior Justice in the majority group). A concurrence is a supplementary or orthogonal explanation from another justice who supports the decision but for a slightly different legal reason. A dissent is an explanation of how a justice on the losing side thinks the law should have been interpreted, or addressing some perceived flaw in the opinion.
The opinion, concurrences, and dissent are (of course) of great interest to lawyers and judges who have to consider similar cases in the future, but it's important to remember that they're not law (lawyers collectively refer to them as dicta, meaning 'speeches' or 'sayings'.). Only the holding is law. I have seen decisions where the holding set precedent and the opinion is so logically or factually flawed as not to make sense, but bear in mind that you can't overturn a court's decision solely by pointing out flaws in the opinion, because the opinion itself is not law. You have to either point a contradiction or falsity within the holding itself (rare) or start over with reference to external sources (Constitution, legislation, prior precedent) to show why the holding is wrong.
It's very important, and often difficult, to distinguish between the holding and the dicta. I mention this because many people, and especially hackers, are liable to get caught caught in some ambiguity or linguistic argument about the contents of an opinion and mistakenly think that they've thereby invalidated the holding. Lawyers and even courts are not immune to this kind of mistake, and cases sometimes come apart because one side has relied too heavily on a particular sentence in an opinion.