The pipes are made of metal or wood. They are lined up in rows in the "organ case" which can be as big as a room. The metal pipes are round tubes. They can be made of different types of metal, but the most common type is an alloy (or mixture of metals) of tin and lead called "spotted metal" because it has round shiny spots on it. This alloy makes pipes sound good as the harder tin provides clarity and definition while the softer lead produces tonal warmth. Very small pipes can sometimes be made of silver, like flutes. Some organs also have some pipes made of brass that sound like trumpets. Most organs have a lot of wooden pipes. The wooden pipes have four flat sides and make a sound different from that of the "spotted metal" pipes. They are not usually seen; they are neatly lined up behind the large metal pipes at the front of the organ which are sometimes painted with colours and patterns. All the pipes have to be made with an end that tapers at the bottom where the air blows in.