The way I have just learned to use the Pen tool, if you hold Cmd/Ctl while selecting the end of a guideline, it will move the guideline on BOTH sides of the anchor point and adjust the last curve that was made. Conversely, if you hold Opt while selecting the same point, it will leave the curve as is and simply adjust the guideline on the same side of the anchor.
In the below example, I find that on the top anchor point, holding Cmd/Ctl while selecting the left end of the guideline, will in this case perform exactly the same function as holding Opt in the above description, while holding Opt will move the entire guideline, i.e. The exact opposite of the above! On the other hand, performing the same actions on the bottom guidelines will cause the expected (above) behaviour...
Does anyone have any idea why this is happening?
There are two types of anchors - corner anchors and smooth anchors.
Smooth anchors always have symmetrical bezier handles.
Command-click-dragging with the Pen Tool on a smooth anchor moves both its' Bezier handles in unison.
Option-click-dragging with the Pen Tool on a smooth anchor handle, and you convert the smooth anchor to a corner anchor.
Corner Anchors may or may not show Bezier handles. If Bezier handles are shown they do not move in unison. There is no symmetry for corner anchor handles. Corner anchor handles move independently of one another.
Command-click-dragging with the Pen Tool on a Corner Anchor handle moves only the clicked handle.
Option-click-dragging with the Pen Tool on a Corner Anchor handle moves only the clicked handle.
There is no difference in operation with the Command and Option keys when they are used on a Corner Anchor Bezier handle.
With the Pen Tool selected, the Option/Alt key is actually a shortcut for the Convert Anchor Point Tool.
When clicking on Bezier handles, the Convert anchor point tool can only break a smooth anchor's symmetry and convert handles to corner anchor handles. If you option-click directly on a smooth anchor, the anchor is converted to a corner anchor without handles. If you option-drag on a corner anchor, you convert the anchor to a smooth anchor with symmetrical Bezier handles.