The 'LINE MODE' allows you to change the line width of the edges of the displayed tree.
Click on the 'WIDTH' button of the 'ARB_NT' window to activate the 'LINE MODE'.
Change line width:
Move the cursor to the desired edge/subtree of the displayed tree and increase or decrease the line width by clicking and dragging the mouse.
Drag the mouse upwards to increase the line width and downwards to reduce it.
Use the right mouse button to change the line widths of all branches of the targetted subtree.
Reset line widths:
Press the key '0' (zero) while the mouse is over a branch to reset the line width of that subtree to the default value.
If no branch is pointed to, the line widths of the whole tree will be reset.