I’m guessing you haven’t been living under a rock all your life and are familiar with the concept of a scale bar. The ScaleBar widget has two main properties that you may want to set: style and unit. The unit property can be set to "metric", "non-metric" or "dual". The style property can be set to "ruler" or "line". Here's an example that shows a map with two ScaleBar widgets:
The only difference I can see between the two styles is that the ruler style has the labels appear above the line while the line style has them below the line. The Esri ScaleBar sample shows that setting the unit to dual will produce a line with the metric label on top and the non-metric label on bottom.