A reel is an object around which lengths of another material (usually long and flexible) are wound for storage. Generally a reel has a cylindrical core and walls on the sides to retain the material wound around the core. In some cases the core is hollow, although other items may be mounted on it, and grips may exist for mechanically turning the reel.

The size of the core is dependent on several factors. A smaller core will obviously allow more material to be stored in a given space. However, there is a limit to how tightly the stored material can be wound without damaging it and this limits how small the core can be.

Other issues affecting the core size include:

• Mechanical strength of the core (especially with large reels)
• Acceptable turning speed (for a given rate of material moving on or off the reel a smaller core will mean that an almost empty reel has to turn faster)
• any functional requirements of the core e.g.
• For a reel that must be mechanically turned the size of the grips that mount it on the mechanical turning device.
• The size of the mountings needed to support the core during unwinding.
• Anything mounted on the cores (e.g. the sockets on an extension reel)

With material such as photographic film that is flat and long but is relatively wide, the material generally is stored in successive single layers. In cases where the material is more uniform in cross-section (for example, a cable), the material may be safely wound around a reel that is wider than its width. In this case, several windings are needed to create a layer on the reel.

Uses

• A fishing reel is used on a fishing rod to wind the fishing line up.
• Kite lines frequently are operated from reels.
• Specialized reels for holding tow line for hang glider, glider, and sailplane launching
• Laying of communications table use giant reels
• Winches wind cables on reels
• Webbing barriers that allow mobile post positions collect tensionally excess webbing.
• Tow trucks hold steel cable on reels.
• Garden hoses reeled solve hose kink problems.
• Rope, wire and cable is often supplied on reels.
• Badge reels are used to hold badges, ski passes and the like.
• A cave diving reel is safety equipment used for running a guideline.

Motion picture terminology

It is traditional to discuss the length of theatrical motion pictures in terms of "reels." The standard length of a 35 mm motion picture reel is . This length runs approximately 11 minutes at sound speed (24 frames per second) and slightly longer at silent movie speed (which may vary from approximately 16 to 18 frames per second). Most films have visible cues which mark the end of the reel. This allows projectionists running reel-to-reel to change-over to the next reel on the other projector.