Getting Started with Nuke - Nuke 101: Professional Compositing and Visual Effects - page 20

Another way to connect a node to the Viewer node's input 1 is by selecting the node you
want to view and pressing the number 1 on the keyboard.
1. Select the Read node by clicking it once.
2. Keep hovering over the Node Graph and press 1 on the keyboard.
Viewer1 now shows the image you brought into Nuke—an image of a run-down
staircase.
FIGURE 1.18 illustrates the Viewer's anatomy. The buttons are explained in more
detail in further chapters.
Figure 1.18. The Viewer's anatomy.