brad,
Do you have either of Jack Sobon's books. Both of those present square rule layout from the edge very well.
The square rule originally used chalk lines because it was originally used on hewn timber. the edges of hewn timber have lots of dips and swells, as well as fat spots and out of square areas. All of which make applying a square to themis too irregular to accurately place a square on.
As sawn timber became more accurate, the need for reference lines went away. If the timber is straight and regular, why snap a straight line? It's a wasted step if the edge is true. For a lot of sawn stock, this is the case. The edges were pretty straight, but the dimensions varied enough to warrant reductions and gains at the joints.
The reason this topic is talking about snap lines so much is because the original poster was asking about how to deal with extreme situations.
