The Schengen Agreement abolishes border controls when traveling between countries in the Schengen area. This means once you enter the Schengen area you are unlikely to need your passport as you cross over into another Schengen area country. This lack of border controls is great for Schengen area locals but includes a limitation for visitors: a three month (90 days) maximum stay in a six month (180 days) period. For an American nomad who wishes to travel for longer terms, this presents the problem of wanting to stay in the Schengen area but being limited to only three months. For example, if you spent two consecutive months in Germany and wanted to spend another two months in France, you wouldn't legally be allowed. If you had already spent two in Germany, you'd only have one month left for France. So how can you get around the three month limitation if you wanted to stay longer in Schengen countries? You can get a visa from a specific country, some of them do offer nomad visas....
The modern nomad's guide to traveling.