Turning Cogs
With two identical cogs, the marked tooth on one cog will always go in the same gap on the other cog.
If the cogs differ in size by one tooth, then eventually, the marked tooth on the smaller cog will fit into each gap in the larger cog in turn.
If the number of teeth on each cog share a common factor, then a marked tooth on one cog will never go in all the gaps of the other cog.