In many instances, the effectiveness of a poem derives from the tension between the use of linguistic and formal units. With the advent of printing, poets gained greater control over the visual presentation of their work. As a result, the use of these formal elements, and of the white space they help create, became an important part of the poet's toolbox. Modernist poetry tends to take this to an extreme, with the placement of individual lines or groups of lines on the page forming an integral part of the poem's composition. In its most extreme form, this leads to the writing of concrete poetry.