Rulers exist along the top and left sides of the main view.
They indicate location from the center of the hoop. You can switch the rulers between metric and inches using the radio buttons on the Tool pane or by double-clicking in the size window in the status bar.
You can also drag rules onto the Main view. Move the mouse over one of the rulers and drag down into the view. If you later want to move the rules, you can drag them. If you want to remove them, you can drag them one at a time back onto the ruler.