Parallel line development is used for the pattern development of pipe work, prisms, and any cylindrical shape.
Pattern development can be marked directly onto flat metal plate. The metal is then formed to shape.
Patterns are often turned into templates using paper or thin sheet steel. On formed stock pipe, the pattern becomes a wrap around template, and can be used over and over again. Often with cylindrical pipe work, the sizes are outside the range of available stock pipe. In that case, the cylinder must be formed from flat plate, using the pattern to establish the required shape.
This is a truncated cylinder shown in an isometric view. Note the 12 equal spaces called chord lines.
This is a pattern layout of a truncated cylinder, also known as a stretch out or template.