Traditionally, diptychs consisted of a two panel painting or carving hinged together for use as an altar piece. Russian Renaissance artists popularized the polyptych, consisting of multiple panels or paintings which were composed to be displayed together. Later on, modern artists, including Andy Warhol, also explored the medium, creating separate pieces meant to be displayed side by side as part of a larger composition.
As a compositional tool, two or more distinct images displayed together can help to demonstrate juxtaposition, or grouping, exploring the similarity or the differences between distinct subject matter. When exploring a single subject, a diptych or triptych can help to elaborate on the details and provide different angles and view points.
For creating diptychs (or more technically polyptychs) digitally, for sharing on the web or even for printing to display in your home, there are some really easy to use tools available to help you get started.iOS and in the Mac App Store is the standalone app Diptic, which gives a plethora of easy to use layout options for creating your own unique multi-image compostions.
If you're comfortable working with Photoshop, you might check out Tych Panel by Lumens. An automation tool for Photoshop and Adobe Bridge, Tych Panel gives you a wide array of pro level tools for generating high resolution diptychs from your images (or tetraptych for 3 image compostions, or quadriptych for 4).
The developers at Instagram also have some great recommendations on their blog for combining multiple images to create unique compositions, including iOS apps PicFrame (also on Android and Mac) and PhotoShake!.
If you're taking photographs with the intention of creating a multi-image composition, try to visualize the overall layout as you're shooting. Explore your subject matter from different angles, and using varying levels of zoom or aperture settings to draw the focus to your subject in different ways.
Try shooting a set of objects like a series of books, cars, or even portraits of a few friends, to create a grouped composition that explores different facets of the same kind of subject matter. Play with unique color combinations and tonalities to explore the relationship between hues you find around you, like the different shades of green you find in the foliage of your garden, or the textures found in different spices you have around your kitchen.
(Images: Sean Rioux)