Useful links when researching vintage patents

European Patent Office — https://worldwide.espacenet.com

My goto resource for vintage patents. Great search facilities and worldwide coverage.

Google Patents — https://patents.google.com

Google's search engine for patents. Quite good, but I prefer the EPO's user interface. Quite often Google links to the EPO website for details.

U.S. Patent Office — https://www.uspto.gov

This site also has trademark information, but the user interface is often confusing and it only covers U.S. documents.