Threat of substitute products or services
The existence of products outside of the realm of the common product boundaries increases the propensity of customers to switch to alternatives. For example, tap water might be considered a substitute for Coke, whereas Pepsi is a competitor's similar product. Increased marketing for drinking tap water might "shrink the pie" for both Coke and Pepsi, whereas increased Pepsi advertising would likely "grow the pie" (increase consumption of all soft drinks), albeit while giving Pepsi a larger slice at Coke's expense. Another example is the substitute of traditional phone with a smart phone.