This is the oldest form of production. With this type of manufacturing, we are concerned with individual products, which will probably not have a large number of pre-manufactured components (many different parts) in them.
Most of the time no two products will be the same and as the units are manufactured to customers' requirements, each unit can be expensive.
Because very small numbers are made, the manufacturer would have to reset the equipment for each individual job. The manufacturer may even have to use different materials for each individual product. All this adds to the production costs for the company. We say that each one of these one-off products has a high unit cost.
The image below is of a child's handmade rocking horse. It may have taken months to make. The horse would be quite expensive to buy because it is a one-off and takes such a long time to make.