If you are using an online gradebook it can be very handy to have your gradebook side by side with the web tools you are using. I had shared in a previous post a Chrome extension that allows you to have windows side by side, but sometimes it is better to do it manually.
If you click on a tab in your browser and hold down the click you can not only drag the tab into new positions, you can pull the tab out to create a new window. (Note: You can also use Command N to create a new window.)
After pulling separating out the tab you can grab the top of the window to reposition the window onto half of the screen. Hovering the mouse over the EDGE of the window will allow you to resize the width. Notice the mouse changes to resizing arrows when you get to the edge. If it is not working you may be in full screen mode, in which case you must first exit full screen mode.
Resize the width of the original windows so that they are side by side.