The following is a transcript of " The Legal Lad's Quick and Dirty Tips for a More Lawful Life, " a podcast from QuickAndDirtyTips.com . The audio program is available via RSS feed here and at TheStreet.com's podcast home page .

Hello, and welcome to Legal Lad's Quick and Dirty Tips for a More Lawful Life.

But first, a disclaimer: Although I am an attorney, the legal information in this podcast is not intended to be a substitute for seeking personalized legal advice from an attorney licensed to practice in your jurisdiction. Further, I do not intend to create an attorney-client relationship with any listener.

Today's topic is the ex post facto clause of the Constitution. Kris wrote:

Recently, the president has been pushing for Congress to update the FISA law to contain "retroactive liability protection" for the telecommunications companies, but isn't this unconstitutional according to Article I, Section 9, which states that "no bill of attainder or ex post facto Law shall be passed."

Very interesting question Kris. The short answer is that the ex post facto clause of the Constitution only prohibits the government from increasing punishment for a something you did in the past, but does not apply to situations where the government decreases punishment.

Article I, Section 9, clause 3 of the Constitution, which applies to Congress, provides that "No Bill of Attainder or ex post facto Law shall be passed." Article I, Section 10, clause 1 similarly prohibits the States from passing ex post facto laws. But what in the world does that phrase mean?

As Justice Chase noted in 1798, the proscription against ex post facto laws "necessarily requires some explanation; for, naked and without explanation, it is unintelligible, and means nothing." Well said.

Ex post facto literally means "from something done afterward." Justice Chase noted four categories of ex post facto laws:

    1) Laws that make an action done before the passing of the law, and which was innocent when done, criminal -- and punishes such action;

    2) laws that aggravate a crime, or makes it greater than it was when committed;

    3) laws that change the punishment, and inflicts a greater punishment, than the law attached to the crime when it was committed, and;

    4) laws that alter the legal rules of evidence, and receives less, or different, testimony, than the law required at the time of the commission of the offense, in order to convict the offender.

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