Employment structures can also change over time within the same country.
In the UK in 1800 most people would have been employed in the primary sector. Many people worked on the land, and made their living from agriculture and related products.
During the industrial revolution, more people were needed to build ships, work in steel-making and with textiles. All of these jobs are found in the secondary sector. By 1900 over half of the workers in the UK were employed in secondary industries.
Since 1900 mechanisation meant that fewer people were required to work on the land and in industry, as machines could carry out most of the work that people previously did.
Foreign industries also became more competitive and imports such as coal became more affordable. As the availability of coal declined in the UK, and also became more expensive to extract, more coal was imported. This led to a further decline in primary sector employment in the UK.
The demand for work increased in schools, hospitals and retail industries. Many people left the rural areas in the search for jobs in the towns and cities. By the year 2000 over half of the UK workforce were employed in tertiary industries and only a small number were employed in primary industries. This has changed the work that people do, and also where they work. Quaternary industries are a relatively new concept, and it is only recently that they have been added to these figures. However it is becoming an important and growing sector in the UK as many firms want to carry out research and development for their products.