Summer is the peak time to visit Prague due to the warmer weather, though spring and autumn are a good option for those happy to trade warmer temperatures for thinner crowds. The city is easily accessible by planes, buses, and trains within Europe. The Prague Metro network is easy to use, consisting of just three lines designated by letters and colors. Since the Czech Republic is a member of the European Union (EU) and part of the Schengen Area, most tourists do not require a visa to visit Prague (just a valid passport, or ID card for EU citizens). The official language is Czech, and the currency is the koruna. Ten percent is the usual tip for good service. As in most of continental Europe, the electricity supply in Prague is 230v.