Split view

The arrangement is older than the web: a file browser, a mail client, and a settings window all put the index on the left and the thing you picked on the right. What makes it a split view rather than two boxes next to each other is the dependency. The leading pane holds a selection, the trailing pane is that selection expanded, and the selection stays visible while you read it, so moving to the next item costs one click instead of a click and a trip back.

Two properties are worth writing down. The panes have their own size rules: the leading one is usually resizable within a minimum and a maximum, and the trailing one takes what is left, which is why the divider clamps in the specimen rather than letting the sidebar disappear. And selection persists: the row you came from stays marked, because a detail pane with no visible source reads as a page that arrived from nowhere.

Apple’s vocabulary allows a third column, which is the same relationship applied twice: mailbox, message list, message. Past three the pattern collapses under its own width, and past two it needs a plan for what happens when the window is too narrow to hold them all. That plan is the same everywhere: the panes stop being side by side and become a stack you navigate, list first, detail pushed in over it, with a back control that reverses the push. Building the narrow case as an afterthought is how a split view ends up with a detail pane nobody can get out of.