Synthetic-filament brushes come in a variety of shapes: flats, filberts, and rounds. Flats are especially useful for painting architecture and machines because they can lay down rectangular shapes easily. You can also turn them sideways to paint a fine line.
Here I'm using a Winsor and Newton Series 995 Golden Taklon ½ inch flat to establish the dark shape of a crypt door.
The finished watercolor painting shows the chunky half-inch stroke module in the stonework and the big shadows. I also used a #4 sable round travel brush for some of the fine line work.
You can use ¼ inch nylon flats with oil or acrylic. For opaque media they come with longer handles. They’re inexpensive, but they don’t last long. Eventually lose their chisel-sharp edge.