There are a couple different reasons someone would want to do a patent search. Probably the most common is someone with an idea wanting to know if it has already been patented. Less common is what I do a lot, trying to find the patent that corresponds to an item marked with a patent date. It’s usually a tool of some sort though at times I’ve been asked to search for a patent when what the item is is in question. I find that the easiest way to find a patent for a known item is to find a similar patent first, then do a search using the classification(s) the similar patent has. If you limit the search to patents that have an issue date matching the date on the item you should only have to look through a handful of patents to find your item. (There is no guarantee it will always work though. One time I was fooled by the patent date on a woodworking tool. It turned out the patent was for the thumbscrew itself – not the tool as I had assumed.)
Here is a page I created for my fellow tool collectors, favoring the USPTO classes where tools are often found. I also have a database of about 8,000 assorted patents that interested me when I came across them. It can be searched to find what uspc’s or cpc’s each patent was classified as and I’ll even generate the USTPO query for your similar item! In my searches I wildcard the subclass to sidestep a maze of twisty passages (there are 400 or more sub classifications in some classes). I’d rather look at a few more patents than spend time navigating class definitions trying to pick likely subclasses to explore.
A real life example: Someone was looking for the patents that correspond to the two patent dates on the back of a grave marker of all things. I’m not as versed in what USPTO classifications grave markers fall under as I am with tool classifications so I searched my database. Sure enough, for reasons that escape me now, I had entered a patent for a grave stone picture holder. I searched the class that that patent was classified under and specified an issue date matching the earlier date on the item and the USPTO returned 18 patents meeting my criteria (ccl/40/$ and isd/2/17/1925). The 16th patent returned was 1,526,381 Grave Marker by Marion Slawson of Girard, KS. A similar search using the later patent date returned 13 patents (ccl/40/$ and isd/12/6/1927). The 7th one was 1,651,780 Grave Marker also by Marion Slawson of Girard, KS. If this was an episode of Myth Busters I’m fairly certain they’d rule it Confirmed after viewing these 31 patents of the 1,784 patents issued on these two dates. Also to support the wildcard assertion, note that there are 615 subclasses in class 40 had I bothered to investigate.
The example above used the USPTO’s classification system but it would work using cpc’s or ipc’s. Find a similar patent in your favorite classification system and then use its classification to find your item. If my database doesn’t have what you are looking for you could try using the USPTO’s web site, they have a few million more patents in their database than I have!
Note that the USPTO has stopped classifying patents under its own classification system and now uses cpc’s. You can still do classifications searches using their system, just be aware that the results won’t include patents issued after June 2015. Another thing I noticed is that not all US patents were retroactively assigned a cpc as the uspto intended. I found 35,454 cpc-less, non withdrawn patents between patent numbers 1 and 4,000,000 so be careful if you are searching for historic ip via cpc! (Here’s a sample page listing a few of the cpc-less patents).