Copying an entry instead of filling in a series
You can also use the Ctrl key to force Excel to copy an initial entry in the cell range you drag through in those situations where Excel would otherwise use the entry as the starting point for generating a sequential series. Here, the association of the Ctrl key with copying in drag-and-drop operations makes perfect sense (reinforced by the appearance of a tiny plus sign that appears above and to the right of the black cross mouse pointer).
You don't have to press the Ctrl key before you drag the range of cells. You can convert a filled range into copies of the initial cell value by pressing Ctrl after you've selected the fill range as long as it's before you release the mouse button. After that, you have to click the AutoFill Options button and select Copy Cells on its pop-up menu to make this conversion.