If it is on a horizontal line:
Y and Guide (with or without friction - it is up to you)
+y, guide with gap -y with gap (with or without friction - it is up to you)
If it is on a vertical line:
Guide (with or without friction, with or without gap - it is up to you)
Regards,
Edited by danb (06/18/12 12:11 PM)
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Dan