Hi Bruce,

here two examples
(you have to copy them into an editor like notepad - sorry for the inconvenience)

both examples have their reversal point in the middle, Line A is considered the interrupt source for Alfredo's example.

A _==__==__==__==_
B __==__====__==__
+ + - -

A _==__====__==_
B __==__==__==__
+ - -

Now you see that the example below is off by one count for the same distance,
this adds up when moving back and forth.
applications that require high precision - like mill tables, automated scanners, chip bonding etc. won't get far when their positioning is off.
(a good digital glitch filter is a must btw)

As i said, for simple applications like a mouse for example this doesn't matter ecause you wouldn't notice if your little rodent counts wrong.

Cheers, Roger