Like many 100+ year old buildings, the walls were uneven with various layers of plaster and sheetrock. Because of the precise alignment needed for the brackets, the wall was opened and solid wood (blocking) was installed between the wall joists to form a solid straight surface and the finish wall was evened out. Our contracto Kenji and I talked about the merits of installing the bracket connection perpendicular to the wood handrail or vertical. Installing the brackets vertical means the wood handrail needs to be drilled at an angle -- a more difficult procedure. We picked vertical. This link shows a cool video of how the bracket is installed.