With cloud services now widely available on the market, flexibility for customers has multiplied. Quickly start a virtual test environment on Azure at the push of a button and then clean it up again. However, if you want to run several virtual machines on Azure continuously, it is worth planning and save costs by means of a reservation. In order not to have to sacrifice flexibility completely, there is the "Azure Reserved Instance size flexiblity" offer, which is used for VMs and scale sets. In this article, I will go into more detail about how Azure Reserved Instance size flexibility works and how you can use it to your advantage and save money.