Necessity, it is said, is the mother of invention. Sometimes, however, invention appears to come first.

How else to explain U.S. patent 3984595, an inflatable rug? Or U.S. patent 05356330, an apparatus for simulating a "high five"?

As Rutherford B. Hayes, 19th U.S. president, is reported to have said after first seeing a telephone in 1876: "An amazing invention - but who would ever want to use one?" It is doubtful the inflatable rug will ever prove as necessary as the telephone.

Nonetheless, it earned the blessing of the U.S. Patent and Trademark Office, along with the high-five device and all manner of strange patents on display at Delphion's Gallery of Obscure Patents. Delphion provides details and images of a number of unusual patents is has collected over the years, from the bird diaper to the human slingshot machine.

(It also has a gallery of historical patents, showing inventions as they were first submitted to the USPTO, including the air bag, the flying machine and Velcro.)

In fact, there is no shortage of wacky patents, judging from the number of Web sites devoted to cataloging and displaying them. For a virtual tour of the odder side of patent law, the following sites can serve as your guides.

• Ig Nobel Prizes. Prizes that celebrate the unusual in science, medicine and technology, the Ig Nobels are awarded to developments "that first make people laugh, then make them think." Patented inventions are among each year's honorees, as are research reports, demonstrations and other scientific developments.
• IP Funny. A blog written by "a couple of bloggers wanting to have some anonymous fun," IP Funny tracks intellectual-property humor. An oxymoron? Not based on some of the items the authors come across, such as the cartoon version of a patent-prosecution flow chart or the condom whose design bears an unsettling resemblance to an ice cream scooper.
• IPWatchdog Museum of Obscure Patents. Eugene R. Quinn Jr., a patent attorney and law professor in Syracuse, N.Y., each week highlights another obscure patent. His discoveries include the buttocks support device, the flush toilet for a dog and the "beerbrella." He keeps past weeks' sites in an archive and provides an RSS feed for those wishing to keep up with his latest posts.
• KinsellaLaw. Stephan Kinsella, a patent attorney and blogger in Houston, Texas, has compiled an index of links to patent law resources. He includes links to some of the funny patents he has come across on the USPTO's site. These include the pat-on-the-back apparatus and the Holy Bible trailer-hitch cover.
• Patently Absurd. This site features patents from the United Kingdom and other international sources. Inventions here include a ladder to enable spiders to climb out of a bath tub and a method of growing unicorns.
• Patently Silly. While IP sites often are written by lawyers, this one is maintained by an engineer turned stand-up comedian. Every week, he sorts through new patents issued by the USPTO in search of ones he considers to be really weird, really cool or really scary. Among his finds: a slide-out deck for a recreational vehicle and a "device for determining and characterizing noises generated by mastication of food." His archive is arranged by topic, making it easy to find silly patents related to, say, Law & Order.
• Patent of the Week. Operated since 1996 and updated weekly, this site has built a collection of more than 200 "strange, interesting, bizarre, inexplicable, wacky, useful and sometimes just plain perverted patents." View them chronologically or by category.
• PATEX's Bizarre Patent Page. This site features Canadian as well as U.S. patents. From north of the border comes the "reality mediator," designed to prevent "the theft of visual attention or theft of mental visual processing resources."
• Purdue Law Offices, Wacky Patents. Patent attorneys in Toledo, Ohio, offer this modest collection of unusual patents, such as the parachute fire escape.
• The Smoking Gun. Known for combing government archives for hidden documents, the folks at The Smoking Gun have compiled two collections of odd patent filings: Plumbing the Patent Files and Inspector Gadget Gizmos. Patents shown here cover inventions such as a wig-flipping device and ergonomic underwear.
• Totally Absurd Inventions. Tracking "America's goofiest patents" since 1997, this site features a new invention each week and archives prior patents by title. True to its name, absurdity characterizes many of the inventions shown here, such as the dummy chicken farmer, a dummy programmed to patrol the coop at fixed intervals.
• Wacky Patent of the Month. Devoted to recognizing remarkable and unconventional patented inventions, these monthly offerings are compiled by Florida patent attorney Edward P. Dutkiewicz. His finds include a device designed to provide an instant face lift and a locket to store chewing gum.
• Wacky Patents. Patents+TMS, a Chicago law firm, offers this small collection of wacky patents, with no updates. It includes the motorized ice cream cone and the toe puppet.
• Wacky Patents and Wacky Gadgets. In compiling this catalog of wacky patents, About.com has organized them by themes. Here, for example, you can find crucifix and Jesus patents, heart-shaped patents and clover and leprechaun patents.
• Weird and Wonderful Patents. Maintained by the Ithica, N.Y., law firm Brown & Michaels, this site tracks what it describes, with tongue in cheek, as "vital advances to the state of the art." It includes the much sought after better mousetrap as well as a vest that lets you wear your pet hamster. This site is not regularly updated.