From the 'how not to properly re-wire an historic home' comes the following example:
On the left the landing light was installed with a flat pancake style round box, which has been screwed into the lath after the plaster was broken away to allow it to fit. These boxes are so small that you can't legally make a junction in them, meaning they always have to be at the end of a wire run. The two half inch #4 screws wouldn't support much fixture weight either. Avoid using these if at all possible.
On the right I have removed the pancake box, cut through the lath, and installed a standard ocagon box which is screwed into a 2 x 4 brace spanning the joists above. There is more space in this box, and it can support a lot more weight as it is now screwed into a structural support brace, rather than just the lath.
. . . . . The Wrong Way . . . . . . . . . . . . . The Right Way . . . . .