Do you mean, you want to get objects perfectly stacked so the top of one just touches the bottom of the next one?

If so, you can do this like this:

• Select them all
• In the align window, switch it to align to key object
• Make sure the numbers box is set to 0
• Hit the distribute objects vertically button in the bottom left of the panel

This puts everything in a stack based on the stacking order , where the distance between each object is the number in the box. If they're inthe wrong order, re-arrange which objects are on top of each other.

Do you mean, you want to get objects perfectly stacked so the top of one just touches the bottom of the next one?

If so, you can do this like this:

• Select them all
• In the align window, switch it to align to key object
• Make sure the numbers box is set to 0
• Hit the distribute objects vertically button in the bottom left of the panel

This puts everything in a stack based on the stacking order , where the distance between each object is the number in the box. If they're in the wrong order, re-arrange which objects are on top of each other.

This stacks any number of objects:

That said, if you're just making simple arrows, there's an easier way using the arrowheads options on the Stroke window. I think in versions earlier than CS6, arrowhead options were buried in the Effects menu somewhere. If you can't see these options, hit "Show options" in the flyout menu at the top right of the box (Adobe love hiding useful tools without leaving any clue that they are there):