Two goods are if the quantity consumed of one increases when the price of the other decreases

Quantity demanded is a term used in economics to describe the total amount of a good or service that consumers demand over a given interval of time. It depends on the price of a good or service in a marketplace, regardless of whether that market is in equilibrium.

The relationship between the quantity demanded and the price is known as the demand curve, or simply the demand. The degree to which the quantity demanded changes with respect to price is called the elasticity of demand.

In economics, quantity demanded refers to the total amount of a good or service that consumers demand over a given period of time.
Quantity demanded depends on the price of a good or service in a marketplace.
The price of a product and the quantity demand for that product have an inverse relationship, according to the law of demand.

The price of a good or service in a marketplace determines the quantity that consumers demand. Assuming that non-price factors are removed from the equation, a higher price results in a lower quantity demanded and a lower price results in higher quantity demanded. Thus, the price of a product and the quantity demanded for that product have an inverse relationship, as stated in the law of demand.

An inverse relationship means that higher prices result in lower quantity demand and lower prices result in higher quantity demand.

A change in quantity demanded refers to a change in the specific quantity of a product that buyers are willing and able to buy. This change in quantity demanded is caused by a change in the price.

An increase in quantity demanded is caused by a decrease in the price of the product (and vice versa). A demand curve illustrates the quantity demanded and any price offered on the market. A change in quantity demanded is represented as a movement along a demand curve. The proportion that quantity demanded changes relative to a change in price is known as the elasticity of demand and is related to the slope of the demand curve.

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Say, for example, at the price of $5 per hot dog, consumers buy two hot dogs per day; the quantity demanded is two. If vendors decide to increase the price of a hot dog to $6, then consumers only purchase one hot dog per day. On a graph, the quantity demanded moves leftward from two to one when the price rises from $5 to $6. If, however, the price of a hot dog decreases to $4, then customers want to consume three hot dogs: the quantity demanded moves rightward from two to three when the price falls from $5 to $4.

By graphing these combinations of price and quantity demanded, we can construct a demand curve connecting the three points.

Using a standard demand curve, each combination of price and quantity demanded is depicted as a point on the downward sloping line, with the price of hot dogs on the y-axis and the quantity of hot dogs on the x-axis. This means that as price decreases, the quantity demanded increases. Any change or movement to quantity demanded is involved as a movement of the point along the demand curve and not a shift in the demand curve itself. As long as consumers' preferences and other factors don't change, the demand curve effectively remains static.

Price changes change the quantity demanded; changes in consumer preferences change the demand curve. If, for example, environmentally conscious consumers switch from gas cars to electric cars, the demand curve for traditional cars would inherently shift.

The proportion to which the quantity demanded changes with respect to price is called elasticity of demand. A good or service that is highly elastic means the quantity demanded varies widely at different price points.

Conversely, a good or service that is inelastic is one with a quantity demanded that remains relatively static at varying price points. An example of an inelastic good is insulin. Regardless of price point, those who need insulin demand it at the same amount.

The income effect in microeconomics is the change in demand for a good or service caused by a change in a consumer's purchasing power resulting from a change in real income. This change can be the result of a rise in wages etc., or because existing income is freed up by a decrease or increase in the price of a good that money is being spent on.

The income effect describes how the change in the price of a good can change the quantity that consumers will demand of that good and related goods, based on how the price change affects their real income.
The change in the quantity demanded resulting from a change in the price of a good can vary depending on the interaction of the income and substitution effects.
For inferior goods, the income effect dominates the substitution effect and leads consumers to purchase more of a good, and less of substitute goods, when the price rises.

The income effect and substitution effect are related economic concepts in consumer choice theory. The income effect expresses the impact of changes in purchasing power on consumption, while the substitution effect describes how a change in relative prices can change the pattern of consumption of related goods that can substitute for one another.

Changes in real income can result from nominal income changes, price changes, or currency fluctuations. When nominal income increases without any change to prices, this means consumers can purchase more goods at the same price, and for most goods, consumers will demand more.

If all prices fall, known as deflation and nominal income remains the same, then consumers’ nominal income can purchase more goods, and they will generally do so. These are both relatively straightforward cases. However in addition, when the relative prices of different goods change, then the purchasing power of consumer’s income relative to each good changes—then the income effect really comes into play. The characteristics of the good impact whether the income effect results in a rise or fall in demand for the good.

When the price of a product increases relative to other similar products, consumers will tend to demand less of that product and increase their demand for the similar product as a substitute.

Normal goods are those whose demand increases as people's incomes and purchasing power rise. A normal good is defined as having an income elasticity of demand coefficient that is positive, but less than one.

For normal goods, the income effect and the substitution effect both work in the same direction; a decrease in the relative price of the good will increase quantity demanded both because the good is now cheaper than substitute goods, and because the lower price means that consumers have a greater total purchasing power and can increase their overall consumption.

Inferior goods are goods for which demand declines as consumers' real incomes rise, or rises as incomes fall. This occurs when a good has more costly substitutes that see an increase in demand as the society's economy improves. For inferior goods, the income elasticity of demand is negative, and the income and substitution effects work in opposite directions.

An increase in the inferior good’s price means that consumers will want to purchase other substitute goods instead but will also want to consume less of any other substitute normal goods because of their lower real income.

Inferior goods tend to be goods that are viewed as lower quality, but can get the job done for those on a tight budget, for example, generic bologna or coarse, scratchy toilet paper. Consumers prefer a higher quality good, but need a greater income to allow them to pay the premium price.

Consider a consumer who on an average day buys a cheap cheese sandwich to eat for lunch at work, but occasionally splurges on a luxurious hot dog. If the price of a cheese sandwich increases relative to hotdogs, it may make them feel like they cannot afford to splurge on a hotdog as often because the higher price of their everyday cheese sandwich decreases their real income.

In this situation, the income effect dominates the substitution effect, and the price increase raises demand for the cheese sandwich and reduces demand for a substitute normal good, a hotdog, even if the hotdog's price remains the same.

The income effect is a part of consumer choice theory—which relates preferences to consumption expenditures and consumer demand curves—that expresses how changes in relative market prices and incomes impact consumption patterns for consumer goods and services. In other words, it is the change in demand for a good or service caused by a change in a consumer's purchasing power resulting from a change in real income. This change can be the result of a rise in wages etc., or because existing income is freed up by a decrease or increase in the price of a good that money is being spent on.

The substitution effect is the decrease in sales for a product that can be attributed to consumers switching to cheaper alternatives when its price rises. A product may lose market share for many reasons, but the substitution effect is purely a reflection of frugality. If a brand raises its price, some consumers will select a cheaper alternative.

Normal goods are those whose demand increases as people's incomes and purchasing power rise. As such, a normal good will have a positive income elasticity of demand coefficient but it will be less than one. This means that a decrease in the relative price of the good will result in an increase in quantity demanded both because the good is now cheaper than substitute goods, and because the lower price means that consumers have a greater total purchasing power and can increase their overall consumption.