A device that lets you insert a plug into a different socket is an adapter: these are small, cheap and safe. For example, between Britain and Germany, you need only an adapter. You stick your British plug in the adapter, which connects the rectangular phase/live and neutral prongs to the round German ones and puts the ground where the German outlet expects it. Then, you're good to go.
Unfortunately, there are many different plugs in the world. The three most widespread standards are the following:
The "American" (Type A) plug, with two vertical pins
The "European" (Type C) plug, with two round pins
The "British" (Type G) plug, with three rectangular pins
If your device has one of these plugs and you can adapt it to the others, you have 90% of the world covered. (The main exceptions are South Africa, Australia, New Zealand, Argentina, and parts of China, which use a Type I plug with two slanted pins.) Adapters between Type A and Type C and from C toG are tiny and cheap; converting Type A into G or Type G into anything else, on the other hand, needs a bulkier model.
For hobbyists: if you can't find an adapter, and you're staying for a longer time, just buy a separate plug at your destination, remove the existing plug, and attach the new one. Unlike adapters, plugs are always available, and they're generally cheaper too. Caution: try this only if you know what you're doing! (Fire and/or electrocution are possible if you are inexperienced.)
As a last resort, a Type C plug can be forced into a Type G socket without any converter at all if you ignore what your mother told you and stick a pen or similar pointy object into the center (ground) hole, which fools the socket into thinking a ground pin has been inserted and opens up the other holes. (This is, in fact, exactly what cheap C-to-G adapters do.) Disable the power to the socket and try to use something non-conductive (a dry non-metallic object) to do this! This procedure will damage the socket and could be unlawful in some countries, so expect the owner to be displeased.
There's one more complication to consider: any two-pin socket is ungrounded, but all three-pin plugs are grounded. Trying to get grounding to work makes life more difficult, as any of sockets C, D, E, F, H, J, K or L will happily accept the ungrounded plug C but will not work with any grounded variant other than their own. Do not use an adapter to turn a three-pin into a two-pin: this will disable grounding, potentially leaving you vulnerable to electrocution and other electrical nastiness.
A last word of warning: many developing countries use multi-plug sockets that accept (say) both Type A and Type C. Don't assume the voltage is correct just because the plug fits, since a Thai Type A+C socket still carries 220V and may destroy American (110V) Type A devices.
The Most Frequently Asked Travel Questions about Electrical systems
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