Hidden detail occurs when a feature of an object cannot be seen in one or more views. For example, we know the cup below is hollow, we can see this in the plan. To show that the cup is hollow in the Elevation and the End Elevation we add dashed lines.

Dashed lines are drawn as shown (the dashes are usually 3mm long). They meet continuous lines without any gap, as shown below.

Dashed lines are the second most important lines we used in Technical Drawing. The only lines that are more important are continuous thick lines. When a dashed line runs into a continuous thick line we leave a gap. An example of this scenario is shown below.

When hidden detail lines changes direction, you continue the dashed line as shown below.

Finally, when two hidden features meet you intersect the dashed lines as shown below.