A population pyramid is a graphical representation of the age and sex distribution of a population. It is a type of bar chart that shows the percentage or absolute number of individuals in each age group and gender.  

The horizontal axis represents the age distribution of the population while the vertical axis represents the percentage or number of individuals in each age group.  

The pyramid shape is formed when the younger age groups are represented at the bottom and the older age groups at the top, with males on one side and females on the other. Population pyramids are often used to analyze demographic trends and to make projections about future population growth and changes. 

• Expanding Population: An expanding population is a population that is growing in size over time. It can occur due to various factors such as high birth rates, low mortality rates, and immigration.  

When an expanding population is represented in a population pyramid, it typically takes the form of a pyramid with a broad base and narrowing towards the top (bell-shaped), indicating that there are more younger individuals than older individuals in the population.  

As the population continues to grow, the pyramid will gradually shift upwards, reflecting the increased number of individuals in each age group. The shape of the pyramid can also provide insights into the future demographic trends of the population. 

• Constant Population: A constant population pyramid is a graphical representation of a population's age and gender distribution where the number of individuals in each age and gender category remains constant over time. This type of pyramid is also known as a stationary population pyramid. 

In a constant population pyramid, the shape of the pyramid is usually rectangular or almost rectangular, with roughly equal numbers of individuals in each age group. This shape indicates a stable population where the number of births, deaths, and migration remains consistent from one year to the next. 

A constant population pyramid is uncommon in reality, as most populations experience some level of change over time, whether it be due to changes in fertility rates, mortality rates, or migration patterns. However, a constant population pyramid can be useful as a reference point for analyzing demographic changes and projecting future population trends. 

• Declining Population: This type of pyramid is also known as an inverted or upside-down pyramid. 

In a declining population pyramid, the shape of the pyramid is wider at the top than at the bottom, indicating a larger proportion of older individuals than younger ones. This shape is a result of low fertility rates and increased life expectancy, leading to a decreasing number of births and an aging population. 

A declining population pyramid can have significant implications for a country's economy, healthcare, and social systems. An aging population can strain healthcare systems, increase the demand for retirement benefits, and reduce the number of working-age individuals to support the economy. 

Governments can use information from declining population pyramids to plan for future challenges and implement policies to address the needs of aging populations. Examples of such policies include increasing retirement age, promoting immigration, and increasing investments in healthcare and social welfare programs.