Ordinarily, clicking the Play button plays your project from the first frame until the last. However, you can change the play range of your project by modifying the In and Out points in the Timeline ruler or by using Menu commands. You might do this to focus on a specific section as you fine-tune your project or make other changes to it. When you finish, reset the In and Out points to the beginning and end of your project.
Do one of the following:
In the ruler, drag the In point marker from the left edge of the ruler to the frame where you want to set the In point. As you drag, the playhead moves with your pointer. When you release the mouse button, the playhead snaps back to its previous position.
In the ruler, move the playhead to the frame where you want to set the In point, then choose Mark > Mark Play Range In.
In the ruler, move the playhead to the frame where you want to set the In point, then press Option-Command-I.
Do one of the following:
In the ruler, drag the Out point marker from the right edge of the ruler to the frame where you want to set the Out point.
In the ruler, move the playhead to the frame where you want to set the Out point, then choose Mark > Mark Play Range Out.
In the ruler, move the playhead to the frame where you want to set the Out point, then press Option-Command-O.
Do one of the following:
Choose Mark > Reset Play Range.
Press Option-X.
The In and Out points reset themselves to the beginning and end of the project.
Do any of the following:
Navigate to an In point: Choose Mark > Go to > Play Range Start (or press Shift-Home).
Navigate to an Out point: Choose Mark > Go to > Play Range End (or press Shift-End).