The cartoon is transferred and Verdaccio (monochrome underpainting) is in progress. In Fresco, the color is not just smeared thick over the surface like artist that knows little is able to do with other mediums. The color is "built" by underpainting of values and form with transparent color building the depth and saturation. Classic oil or tempera painting adopted this technique and employ it via three stages; imprimatura - underpainting - dead layer.

Never mind those little drips just below the faint charcoal line - plaster will be scraped off up to that line at the end of the day (look at the picture below). Thin paint drips and due to that frescoes are always painted from the top down.
 

You see the drips are gone so as the access of the plaster - plastering more than a foot over then what is to be painted seems like an awful waste, however since the one has to paint on wet plaster and plaster begins to dry as soon as it is on the wall it is essential to make sure that the edge will not dry into the painting. Leaving enough plaster between its edge and the actual edge of the giornata will create the necessary barrier.