Brand extensions play an important part for brands when approached properly. The most important aspects are the compatibility with the established brand, the opportunity, and leverage (Klara, 2013). Logical fit is the key.
A logical fit, like ZZZQuil is important. Nyquil has been used for sleep during colds and flu, but people were also using it to sleep at other times. So a natural extension was to create the brand extension for those who need help sleeping without all the added cold related ingredients. This brand extension used its established branding (note the quil in the name), saw ...
An argument for the use of brand extensions as well as an explanation of why and how it should be done.