I've always wondered this.

What does it actually mean when you say "Something under the lines of..."

For instance if you were commissioning a sculpture you might sit down with the artist and a pen and paper and say "I want something under the lines of this" and then draw a sketch.

Or if you were describing a book you wanted you might say "The plot was something under the lines of a murder and a police man".

Where does this phrase come from, and what are you really saying?

Thanks (sorry for terrible examples! :p )