A “lesser included offense” is defined as

A crime that is composed of some, but not all, of the elements of a more serious crime and that is necessarily committed in carrying out the crime.

Black’s Law Dictionary

Battery, for example, would be a lesser included offense for the crime of murder.

When a Massachusetts criminal defendant is charged with a lesser included offense, the trial judge can give jurors the following instruction:

You may find the defendant guilty of [the lesser included] only if you are NOT convinced beyond a reasonable doubt that the defendant is guilty of [the greater offense], and you ARE convinced beyond a reasonable doubt that the defendant IS guilty of [the lesser included offense].

Instruction 2.280

Thus, jurors can find the defendant guilty of the lesser included offense while acquitting him of the greater offense.

The trial judge must give such an instruction to the jury whenever there is a rational basis for doing so and either the defendant or the prosecutor has requested it.

As noted by the Supreme Judicial Court,

when the evidence permits a finding of a lesser included offense, a judge must, upon request, instruct the jury on the possibility of conviction of the lesser crime” (emphasis supplied).

Commonwealth v. Hobbs, 385 Mass. 863, 871 (1982).

Despite this, a trial judge may, on his own initiative (sua sponte), give the “lesser include offense” instruction to the jury.

If a charge on a lesser included offense is appropriate, the defendant cannot insist that the instruction not be given. See Commonwealth v. Pizzotti, 27 Mass. App. Ct. 376, 384-385 (1989) (“A judge need not fall in with a defendant’s desire to gamble on the jury’s acquitting him of the larger crime if that crime were put to them without the other choice”).

Commonwealth v. Thayer, 35 Mass. App. Ct. 599, 603 n.9 (1993).

In terms of double-jeopardy, a lesser included offense and the greater offense are considered the same crime.  Consequently, acquittal or conviction of either offense precludes a new trial for the other.