Types of bulbs
There are three main types of bulbs according to international regulations. These rules are determined by UN Regulation 37, which specifies which types of bulbs
may be used in vehicles and where they can be usd. Regulation 37 devides bulbs into Group 1, Group 2 and Group 3. This does not include HIDs, which are governed by a separate rule.
Group 1 bulbs are those that may be used for anything. These are typically your common headlight bulb fittings. Most bulbs in this category start with the letter H - H1, H4, and H7 are included in Group 1 and are popular headlight bulb fittings. There are a few exceptions, such as S2 or S3 bulbs, which are used in mopeds or motorcycles.
Group 2 bulbs can be used in almost any application except as headlights. This category includes a very wide range of bulbs, including signalling lights, brake lights, interior lights, sidelights, or number 
plate lights.
Finally, Group 3 includes bulbs that are no longer allowed to be produced on new vehicles. These bulbs can, however, be manufactured as replacement parts.
W5W bulbs
Where does the W5W fit in? It falls under Group 2. This means that the bulb can be used for all types of applications, but not in headlights. A W5W is commonly used in interior lights and sidelights. Some vehicles use them for number plate lights, too.
The name W5W means that the bulb is 5 watts and has a wedge-type base. You can find both halogen and LED bulbs in this fitting. If you're using W5Ws on the exterior of your car, such as in sidelights, you may want to upgrade to LED. If you do this, you'll benefit from a longer-lasting, brighter bulb. They also make a great upgeade for your interior, giving it a cooler, more modern look.
When purchasing a W5W for your vehicle's exterior, be sure to check whether it's road legal. There are many W5Ws, including LEDs, that are designed for interior use only. These have a cooler colour temperature and are not road legal when fitted in the exterior of your car.
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