There are some in-stream ads that are non-skippable and can play in many different sections during the partner video. These are called pre-roll ads and they can be shown in the beginning, middle or end of a video and are between 15 to 20 seconds long. Due to the fact that pre-roll ads cannot be skipped, these videos are best created with a call-to-action or CTA, this gives you the chance to better optimize the viewer that you do receive. To better optimize, means to encourage the viewers to click on your ad and potentially receive something in return.