When you draw sketch geometry constraints are automatically added, but there are a couple of ways to turn this feature off.
This is called Constraint Persistence. In this case a Perpendicular constraint will be added to the new vertical line, and it will be perpendicular to the horizontal line.
You can turn on or off Constraint Persistence by clicking the down arrow on the Constrain panel.
When Constraint Persistence is highlighted like this it’s on. Click it to turn it off.
Even though Constraint Persistence is turned off inferred constraints will appear as you draw. The difference is the constraints will not be added to the lines. So the top horizontal line will not be horizontally constrained.
One reason you may want to do this is…
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