Its logical to do one wall at a time but it doesn t work out that way.
Tiling walls that are not plumb.
The horizontal line assures the 3 walls are level with each other and the grout joints will line up wall to wall.
Check the wall first to see if it is straight.
Do this prior to any tiling job.
Slide a 4 foot straightedge or level over the wall in all directions to check for plumb.
Subway tiles are notorious for not being well made so i always tile all 3 walls at the same time so the grout joints match up on adjoining walls.
Answer answer what should have been done while installing the hardibacker board on the walls is the framing studs should have been shimmed and adjusted to make sure the hardibacker board was plumb and square.
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