I didn't know what the ghost ring was until my favorite LBS tech was showing off. A very cool solution, but, got voted down by "less forward thinking"
mechanics. This bike will go to someone who likely wouldn't know what to do if it fell off.
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If you have vertical dropouts & can't slide the axle back & forth, but do have a selection of chainrings /cogs around you can install a "ghost ring" to take up the slack. It looks hack-y but, ghost rings work."