Judges often have discretion to decide whether to give defendants who are convicted of separate crimes concurrent or consecutive sentences. (See Oregon v. Ice, 555 U.S. 160 (2009).) The decision can come up when the defendant is convicted of (or pleads guilty to) multiple offenses in the same case (see Example 1, below); or when the defendant is already serving time for a conviction, but is tried later for another crime (see Example 2, below).
Concurrent sentences. When sentences run concurrently, defendants serve all the sentences at the same time.
Consecutive sentences. When sentences run consecutively, defendants have to finish serving the sentence for one offense before they start serving the sentence for any other offense.
If a defendant is convicted of a number of crimes that carry lengthy prison terms, the difference between consecutive and concurrent sentences can be tremendous. The same factors that judges tend to consider when deciding on the severity of a sentence (for example, a defendant's past record) also affect their decisions on whether to give concurrent or consecutive sentences. Some criminal statutes, however, require that the sentence for the crime in question be served consecutively to any other crime committed in the same incident.
Example 1: Haydn Goseek was convicted of 20 counts of forgery for forging and cashing 20 separate checks. Each count carries a maximum possible prison term of five years. If the judge gives Haydn a maximum sentence on each count and runs the sentences consecutively, the total sentence would be 100 years in prison. If the judge runs the sentences concurrently, Haydn's total sentence would be five years in prison because he would serve all of the sentences at the same time. (Whether he receives consecutive or concurrent sentences, Haydn might be released early on parole.) If Haydn previously had a clean record and forged the checks when he had been temporarily laid off from work, the judge might well sentence him to less than the statutory maximum of five years on each count, and run the sentences concurrently. (For information on unlawfully "disproportionate" sentences—perhaps like 100 years for bad checks—see The Meaning of "Cruel and Unusual Punishment.")
Example 2: Same case. Haydn's forgery conviction was in Michigan. At the time of the Michigan conviction, Haydn was already serving a sentence in Indiana for forgeries committed there. (Indiana turned Haydn over to Michigan temporarily to stand trial.) When the Michigan judge sentences Haydn on the Michigan forgeries, Haydn's attorney can ask the Michigan court to allow Haydn to serve the Michigan sentence concurrently with the Indiana sentence. If the judge agrees, every day that Haydn serves in Indiana will count as though it were served in Michigan.
Sometimes, a sentencing judge can legally give just a single sentence to a defendant who is convicted of separate crimes. This comes up when the state has forbade "double punishment" for convictions that result from a single unlawful act. For example, assume that a defendant sets a house on fire in an attempt to kill the occupants. The defendant may be convicted both of arson and attempted murder, but could probably be given only a single sentence. Typically, the sentence would be for the more serious crime, which in this instance would probably be attempted murder. See, for example, California Penal Code Section 654(a): "An act or omission that is punishable in different ways by different provisions of law shall be punished under the provision that provides for the longest potential term of imprisonment, but in no case shall the act or omission be punished under more than one provision."