Could it be the definite article, but omitted? Like in the following case in an instruction:
Grasp drumstick. Place knife between thigh and body; cut through skin
to joint. Separate thigh and drumstick at joint.
All those omittions would normally have the definite article, but this doesn't seem plausible in my case.
Why is there no article here, too?
I've noticed that this happens only with the following prepositons:
before; after; from; to || day after day; from person to person; from teacher to student
My questions are: Why are the articles omitted in all those examples? Does it have something to do with comparisons? Is there any rule for this usage?