In the last two hundred years there have been great changes in the method of production of goods. This is now also true of the building industry; for mechanization has been introduced. System building can save both time and money. The principle of system building is that the building is made from a set of standard units. These are either made at the building-site or at a factory. Some designers, in fact, are standardizing the dimensions of rooms. They are made in multiples of a single fixed length, usually ten centimeters. This is called a modular (标准件的) system, and it means that manufactures can produce standardized fittings at a lower cost. The most important fact about system building is its speed. A ten-storey flat, for example, can be completed in four months. There are several new methods of system building. One is the panel method. In this case, the construction company sometimes erects a factory on the site. The walls and floors of the building, called panels, are cast in a horizontal or vertical position. Conduits for electrical wires and sleeves for pipes are cast in the panels when they are being made. The moulds for these castings are situated all around the building. After the concrete panels are cast, they are allowed to set and harden for a week. Next they are lifted by a tower crane on to any section of the building. There the panels are cemented together at their joints and the floor covering is laid. After the panels have been cemented together, the crane lifts a case into the area. It contains all the fittings to be installed, such as wash-basins, radiators and pipes. Finishing tradesmen, such as plumbers, plasterers, painters and electricians, follow behind to complete the work. In some building developments, in some countries, whole flats with internal features like their bathrooms, bedrooms and connecting stairs, and weighing as much as twenty tons, are carried to the building-site ready-made. A giant overhead crane is used to lift them into position. In the future, this method may become more widespread. The main difference between panel method and the method discussed in the last paragraph is_______.