Tick marks

Tick marks are the printed scale on a slider: a row of small notches along the track saying where the values are. Without them a track is a continuous stretch of nothing in particular, and the reader has no way to tell whether the control accepts every value in between or only five of them. That is the whole job. Tick marks are a scale drawn on a slider, and the slider is the control; naming them separately matters because the marks can be added or removed without changing anything else about how the control behaves.

The most common misreading is that marking implies snapping. It does not. A slider can print a scale and still accept every value between the marks, in which case the notches are a ruler: they tell you where 25 is so you can aim at it, and nothing stops you resting at 27. Making the handle land on the marks is a separate decision, and the moment you take it the marks stop being a scale and become detents, which is the word borrowed from mechanical catches that hold a part at fixed positions. The two go together often enough that the same drawing usually means both, which is exactly why it is worth saying which one you mean.

Density is where tick marks go wrong. A hundred notches on a three hundred pixel track is a grey smear that reads as texture, not as information, and at that point the marks are worse than none. The usual repairs are to print only the major stops, to draw a minor mark shorter than a major one so the eye can group them, or to drop the marks entirely and let a numeric read-out carry the precision instead. Material draws them as stop indicators inside the track and lights the ones behind the handle, which is a neat trick: the marks the reader has passed take the filled colour, so the scale doubles as a sliding indicator of where the value sits.

The detent idea is not confined to sliders. Scroll snap is the same catch applied to a scroller, the resting positions of a bottom sheet are detents by another name, and a rotate gesture that clicks to the nearest 15 degrees is doing what a marked slider does. Tick marks are simply the case where the stops are printed where you can see them, which is why the word turns up in interfaces that want to feel like instruments: cassette futurism is full of them, because a scale engraved next to a control is what hardware does instead of a tooltip.