In marketing jargon, product lining refers to the offering of several related products for individual sale. Unlike product bundling, where several products are combined into one group, which is then offered for sale as a units, product lining involves offering the products for sale separately. A line can comprise related products of various sizes, types, colors, qualities, or prices. Line depth refers to the number of subcategories under a category. Line consistency refers to how closely related the products that make up the line are. Line vulnerability refers to the percentage of sales or profits that are derived from only a few products in the line.

In comparison to product bundling, which is a strategy of offering more than one product for promotion as one combined item to create differentiation and greater value, product lining consists of selling different related products individually. The products in the product line can come in various sizes, colours, qualities or prices. For instance, the variety of coffees that are offered at a café is one of its product lines and it could consist of flat white, cappuccinos, short black, lattes, mochas, etc. Alternatively, product line of juices and pastries can also be found at a café. The benefits from having a successful product line is the brand identification from customers which result in customer loyalty and multiple purchases. It increases the likelihood of customers purchasing new products from the company that have just been added into the product line due to the previous satisfying purchases.

In marketing, the number of product lines offered is referred as the width of product mix. Product mix, also known as product assortment, is the total number of variety of products that a firm sells to their customers. It measures the total number of product lines. Some companies will focus solely and sell only one type of product that they specialise in. Also, some would offer numerous types of products for diversified markets, depending on the size and objectives of the entities. Each approaches' results vary based on many factors including location, market, trends, etc. Therefore, businesses should carefully consider their product mix. The width of product mix is one of the four dimensions of product mix along with the length, depth and consistency of product mix.

As mentioned above, the width of product mix is referred to as the total number of product lines that the company offers. A diversified product mix can target the maximum number of customers, however, such numbers of product lines requires much attention and focus as each product line targets different groups of consumers and involves individual strategy and management. Although specialisation of products (narrow product mix) might be easier for businesses to operate and manage, it reduces the ability to reach out to diverse markets as they fail to offer sufficient options for consumers to cater to their "needs and wants."

The length of product mix refers to the total number of products sold by a company. A product line consists of many similar products defined by its functions and customer market while short product line consists of fewer related products. Customer satisfaction could be achieved through longer product lines. However, overly dense product lines may result in competition within the same line and lead to loss of revenue and customers. If product lines are too short, consumer options are limited, forcing them to switch to competitors with a longer range of products.

The depth of product mix pertains to the total number of variations of product in a product line. For example, a brand would be considered to have a depth of four if it sells two sizes and two flavours of soda.

The consistency of product mix refers to how closely associated the products in the same product line are to each other, in terms of their use, production and distribution. A business’ production mix could be very consistent in distribution, yet extremely different in other areas such as use. For instance, a company may be selling health related items such as multi-vitamins tablet and magazines. Although both products fit into the same product line, they are completely dissimilar in use while one is editable and the other is not.

When companies add a new item to a product line, it is referred to as the product line extension. The purpose of it is to attract new customers who may not be familiar or satisfied with the current standard product line. For example, when a lifestyle pharmacy decided to add in a high protein muesli bar into its current product line of muesli bar. Companies with an effective product line can employ product line extension in order to reach new demographic customers in different geographic areas.