here’s a pic

http://www1.istockphoto.com/file_thumbview_approve/35306/2/istockphoto_35306_iso_boxcutter_blades.jpg

Such a knife generally consists of a simple and cheap holder, typically flat, approximately one inch (25 mm) wide and three to four inches (75 to 100 mm) long, and typically made of either metal or plastic. Some use standard razor blades, others specialised double-ended blades as in the illustration. The user can manually adjust how far the blade extends from the handle, so that for example the knife can be used to cut the tape sealing a package without damaging the contents of the package. When the blade becomes dull, it can be quickly reversed or switched for a new one. Spare blades are often stored in the hollow knife handle, and can be accessed by removing a screw and opening the handle. This type of tool is known in British English, Australian English, New Zealand English and Dutch as a Stanley knife, after one of the first manufacturers to create this kind of implement. The genuine Stanley knife has a cast-metal body, and comes both in retractable versions and in fixed blade versions which allow no depth adjustment. There have been a number of versions of the classic retractable Stanley No. 99. The current model is the 99E [1]. Fixed blade versions are widely used for handcrafts. The blades for a utility knife come in both double and single ended versions, and are interchangeable with many but not all of the later copies. Specialised blades also exist for cutting string, linoleum and other purposes. Spare or used blades may be stored in the handle.

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