Introduction:
The demand curve is a graphical representation that shows the relationship between the price of a product and the quantity of that product demanded by consumers. It is a fundamental concept in economics and is used to understand consumer behavior and market dynamics. When constructing a demand curve, it is essential to follow certain guidelines regarding the placement of variables on the axes.

Compulsory Placement:
The demand curve is conventionally constructed with price on the y-axis (vertical) and quantity demanded on the x-axis (horizontal). This placement is not arbitrary but is based on several key reasons and principles.

Law of Demand:
The law of demand states that as the price of a product increases, the quantity demanded decreases, holding other factors constant. This inverse relationship is the foundation of the demand curve. By placing price on the y-axis and quantity demanded on the x-axis, the downward slope of the curve reflects the negative relationship between price and quantity demanded.

Price Determination:
Price is an essential factor in determining the quantity demanded. It directly influences consumer behavior and their willingness to purchase a product. By placing price on the y-axis, the demand curve visually represents the effect of price changes on quantity demanded. It allows economists and market participants to analyze the impact of price fluctuations on consumer demand.

Quantity Determination:
Quantity demanded is a dependent variable that responds to changes in price. By placing it on the x-axis, the demand curve showcases the relationship between price and quantity demanded. It helps in understanding the quantity of a product consumers are willing and able to purchase at different price levels.

Visual Representation:
The conventional placement of price on the y-axis and quantity demanded on the x-axis allows for a visually intuitive representation of the demand curve. The downward-sloping curve clearly illustrates the relationship between price and quantity demanded, making it easier to interpret and analyze.

Conclusion:
The placement of price on the y-axis and quantity demanded on the x-axis in a demand curve is compulsory due to several reasons. It aligns with the law of demand, highlights the impact of price changes on quantity demanded, and allows for a visually appealing representation of the relationship. By following these guidelines, economists and market participants can effectively analyze consumer behavior and make informed decisions in various industries.