In 2003, when I rebuilt my kitchen, I converted my single towel rail to Twin Towel Rails. They are in a convenient place under the front edge of a worktop. One which is too shallow (front to back) to have any cupboards under it. It has a double panel radiator under it instead.
A single rail was always a nuisance because towels would slip off it regularly. This was due to lack of friction with the rail when they had a weight imbalance due to overhanging one side more than the other.
I recently had a ground floor extension built which required access to the loft space in the pitched roof to provide and service pipes and cables. So I asked the builder to create an insulated loft trapdoor. I made it clear that it should not just be a sheet of MDF or plywood. It must have thermal insulation fixed to it to prevent it forming a cold spot in the ceiling. I also required it to be out of sight if possible. By that I meant he must avoid putting it in the main reception room of the extension.
By widening the gap between two ceiling joists he made an opening 485mm wide. Enough to accommodate a telescopic ladder I bought to climb into the loft.
The builder built the opening and finished it with architrave around it and clapping strips on all sides to form a lip that supports the insulated loft trapdoor. I offered to make the insulated loft trapdoor myself since it wasn’t part of the original estimate. That allowed the builder to get on with work that was part of the original estimate.
I would say sighting a straight line is a common practice used by builders and joiners to determine if a piece of timber is straight or bowed. It can be used to test any length of timber, but long pieces in particular, and requires no tools just eyesight with a good depth of field enabling focusing along the whole length of timber to be checked.
This link goes to Titan Metal Werks, Inc of Ilinois, USA a company which makes SplitStop screws which they claim won’t split the wood they are screwed into. Designed for fixing decking they appear to drill themselves in and make a countersunk hole to boot.
See their example video which I have embedded below. It shows how an ordinary screw splits the wood when it is screwed in. Then it shows how their screws can be screwed in around it, very close to the edge, without splitting the wood:
This is what TITAN have to say about their products:
“Experience the ability to place woodscrews within 1/8″ of the edge of a board without splitting the wood. Install deck boards and railing balusters with little fear of splitting! Penetrate knots without shearing-off screw heads! Countersink effortlessly, even in Oak! All without predrilling!” — Titan