The main types of airship are non-rigid, semi-rigid, and rigid. Non-rigid airships, often called "blimps", rely on internal pressure to maintain their shape. Semi-rigid airships maintain the envelope shape by internal pressure, but have some form of supporting structure, such as a fixed keel, attached to it. Rigid airships have an outer structural framework that maintains the shape and carries all structural loads, while the lifting gas is contained in one or more internal gasbags or cells. Rigid airships were first flown by Count Zeppelin and the vast majority of rigid airships built were manufactured by the firm he founded, Luftschiffbau Zeppelin. As a result, rigid airships are often called zeppelins.