You're absolutely right: https://www.youtube.com/watch?v=lORIZ1shRIM https://www.youtube.com/watch?v=lORIZ1shRIM Given that mortar is semi-solid, there's no wa

You're absolutely right: https://www.youtube.com/watch?v=lORIZ1shRIM https://www.youtube.com/watch?v=lORIZ1shRIM Given that mortar is semi-solid, there's no way you could ever reliably get the same thickness of mortar without an external reference. Older tall chimneys are often visibly crooked in parts. I'm no chimney builder, but it's easy to imagine it being difficult to get the alignment perfect when you're trying to make an angled structure, high up in the air, using primitive tools. (remember that tall chimneys are narrower at the top than the bottom to save money, so you can't directly compare it to a vertical plumb-bob line: you have to offset each layer slightly from vertical) Equally, once the error builds up to the point where you can see it from the ground, getting the rest of the chimney back to true would be easy: just lay the next layers of bricks slightly offset until it looks right again. Results in an slightly ugly looking chimney. But a lot cheaper and faster than demolishing the crooked bit and starting over.