Split button

A split button is a bet that one of the choices is the right one almost every time. The main half carries that choice and runs it in a single press; the attached arrow admits the others exist. Save, with save as a copy and save as a template behind the arrow, is the archetype, and so is Send with schedule send beside it.

The bet fails in two ways. If there is no dominant action, the control invites a coin flip: people press the big half because it is big, and get something they did not want. If the menu holds unrelated commands rather than variants of the default, the arrow stops being a modifier and becomes a second control wearing the first one’s clothes. A menu button is the honest answer to both, because it promises nothing until it is opened.

Drawing it is a matter of making two targets read as two. The seam between the halves has to be visible, each half has to take hover and press state on its own, and the arrow half needs enough width that a fingertip cannot miss it into the action. Under the hood it really is two buttons inside one group: the main one labelled with the action, the arrow one labelled for what it opens and carrying aria-haspopup="menu" with aria-expanded.

Names for it wander. Cloudscape calls the pattern a button dropdown, Fluent calls its version a menu split button, and plenty of teams just say dropdown button, which collides with the plain menu button that has no default action at all. If the control you mean has a main action attached to its arrow, “split button” is the word that says so.