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This situation is seen most often on TV legal shows like Perry Mason and Matlock, but is seen from time to time in real life.

The prosecution is trying to show that Y killed X. The usual means are to demonstrate motive, means, and opportunity. All those weight against the defendant.

The "standard" defense is to dispute one or more elements of the foregoing. But in this instance, the defense attorney uses an "indirect" defense; that Z had similar means, motive and opportunity to do the deed, thereby casting "reasonable doubt" on Y's guilt.

The prosecution objects: "Z is not on trial here, only Y."

What are the canons of trial law that govern how far the defense can go to "put Z on trial" in order to defend Y? (Let's say that Z is the prosecution's star witness, but someone who might have been given immunity in exchange for testimony against Y.) Or is it a matter that is largely left to the discretion of the judge?

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Offering up evidence that another person committed a crime is a standard and accepted, although rare, criminal defense strategy.

It is mostly not used very much because most crimes for which defendants are charged were not committed by somebody else, and because most innocent defendants lack the investigative and economic resources to identify enough information about another potential suspect to present such a case credibly.

The defense would have to articulate to the judge that the reason for offering up evidence about Z is to support Y's theory of the case that Z and not Y committed the crime, in order to overcome a relevance objection. But, the defendant had wide latitude to do so in order to create a reasonable doubt that the defendant committed the crime.

In most circumstances, it would be a reversible abuse of discretion to prevent the defendant from introducing evidence to support a theory of this kind.

A similar strategy, for example, was recently used in a preliminary hearing in a case involving a high profile murder in Colorado where a man stands accused to murdering his wife (the man is Barry Morphew) has developed a theory that his wife was murdered by someone else based in part, for example, on the evidence of unidentified DNA near the murder scene allegedly associated with previous crimes in DNA databases.

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