A mesostic is a poem in which a vertical column of letters spells a word and the horizontal lines intersect with those letters to form a complete poem. This is a different type of poem than an acrostic, in which the beginning letters of each line can be put together to spell a message or word. In most cases, the vertical key word is meaningful to the rest of the poem rather than an attempt to hide a message within the text. One of the most famous writers of mesostic texts was a composer named John Cage, who posited several rules for this type of poetry, although these are not necessarily absolute.
The mesostic form of poetry has an interesting history. It was once used by a poet named Jackson Mac Low, who utilized the form to create poems out of existing texts based on random operations. For example, he once used the beginning words from a book as an index word and the text from the book itself to create an acrostic poem over which he had little control. This process in many ways exemplifies the attractiveness of uncommon images that can result from words suddenly lining up together and the appeal, in general, of mesostics and acrostics created in this manner.
Today, a mesostic rarely takes this randomized construction. Instead, the index word helps to inspire the words that surround that key column. When presented to readers, the poem is usually aligned so that the column is straight and vertical, with its letters set apart from the rest of the text in some way. This can be accomplished with capitalization, color, or even highlighting. The visually intriguing presentation of this type of text makes the poetry particularly appropriate for printing, as it relies on visual presentation for its pattern to be seen.
Many people use this type of poetry in order to generate ideas or as a helpful puzzle that guides the images used in the poem. It is also possible to write prose mesostic texts, although the presentation of these texts is more problematic due to the structure of the lines. Some of the rules posed by Cage impact the types of letters that can be used in certain sections of the text, but these rules are not necessary to the form. The term "mesostic" itself is derived from the Greek words for "middle" and "line of verse," so it is usually considered acceptable to complete the poem in any way so long as the key letters fall somewhere in the middle of the line.