“Chain strain” The term I use to describe excessive deflection of the chain when gear combinations exceed the recommended offsets.

My 21-speed bike has a 3 x 7 gear combination.  (3 cogs on the front sprocket, 7 cogs on the rear sprocket/freewheel).

When the front sprocket is on the center cog and the rear sprocket is on the center cog, the chain should be in linear alignment causing the least amount of deflection and wear on the chain. Moving to the smallest cog on the rear sprocket deflects the chain three positions outward from linear alignment, as does moving to the largest cog (inward deflection from linear alignment).

When the front sprocket is on the small cog, and the rear sprocket is on the large cog, alignment is close, but not perfectly linear, it is already slightly deflected. Therefore any movement towards a smaller cog on the rear sprocket increases the deflection, putting additional strain and wear on the chain. The same holds true for the position of the chain on the large cog of the front sprocket. That’s why It is not recommended to deflect more than 3 positions from the end-most cog to relieve the deflection, and excessive wear, of the chain. Make sense?

Front Sprocket, Center Cog, Rear sprocket/freewheel: Use all seven cogs = 7 combinations
Front Sprocket, Small Cog,  Rear sprocket/freewheel: Only use the three largest cogs = 3 combinations
Front Sprocket, Large Cog, Rear sprocket/freewheel: Only use the three smallest cogs = 3 combinations

Total: 13 recommended gear combinations

And: Thirteen gear combinations don’t necessarily equate to 13 different gear ratios either.  Some combinations may overlap.  Sheldon Brown’s site has a gear calculator if you’re interested in determining the number of specific gear ratios on your bike.

And: Ever notice some of the teeth on the cogs of the rear sprocket/freewheel are deformed/bent?  No need for concern, the deformations are normal, factory original.