The interchangeable terms "crop marks" and "trim marks" point to short line segments arranged at the corners of a printed piece of artwork to define where to trim away the outer edges of the paper, leaving only the artwork behind. Most graphics software, including Adobe Illustrator, provides features and functions that automate the placement of these marks so you don't have to create them manually. Illustrator's artboards -- the page-like areas on which you draw -- define the printable area of your artwork, but crop marks only provide guides for hand trimming or commercial print production.
Press "V" to switch to the Selection tool. Click on or marquee around an object or set of objects to make them the active selection in your Adobe Illustrator file. To include an object that you've hidden or locked, press "F7" to reveal the Layers panel, twirl open the disclosure triangle for the layer that contains the artwork, and turn on the visibility eyeball or click off the padlock icon that denotes an object's status.
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Open the "Object" menu and choose "Create Trim Marks" to display an editable set of corner marks at the four corners of the invisible bounding box that defines the overall height and width of your artwork.
Click on or marquee around the crop marks to edit or delete them. To hide them the same way you would hide any Adobe Illustrator object, select them and press "Ctrl-3."
In addition to editable crop marks, Adobe Illustrator also can create these marks as a live effect. Open the "Effect" menu and choose "Crop Marks" to add them. To delete crop marks you create as an effect, select the object to which you applied them, press "F6" to open the Appearance panel and drag the Crop Marks item to the panel's trash can icon.
Japanese-style crop marks use double lines that define a built-in bleed. To create artwork that appears to extend all the way up to the trim line of a page, you extend the ink or toner beyond the trim line in an extra printed area called the bleed, and crop off the excess paper.
If you define crop marks on an artboard and print or export it with the Marks & Bleed option active in the Print dialog box, your output contains two sets of crop marks: the ones you drew or created as an effect, and the ones included in your output.
Crop marks appear in a special color called "Registration." Don't try to use this color for anything except page markings. Unlike other colors, Registration appears on every color plate when you separate artwork into its printing primary colors -- cyan, magenta, yellow and black -- or the spot colors in a piece of art defined by premixed inks.
Information in this article applies to Adobe Illustrator CC 2014, Adobe Illustrator CC and Adobe Illustrator CS6. It may differ slightly or significantly with other versions or products.