This post describes: How a towel wrapped around two rails parallel to each other in a particular way will not easily fall off onto the floor. It shows what a two rail system looks like but not how to construct a particular one in detail.
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.