A good way of helping both the designer and the client visualise where things will sit on a web page is by using a conceptual tool called wireframes.

They show the layout of the page and give a good indication of where all the content, buttons and images will be.

Wireframes are a great way of ensuring that the client and the designer are on the same page way before all the heavy duty hours are put into actually creating the first design concept.

Of course, the level of detail incorporated into the wireframes and the accuracy of the design when compared to what the client wants all hinges on a good design brief.