Offensive linemans usage of hands/holding

Question
I read your previous explanation of holding, but my question is more detailed that I couldn't conclude from your response.

I have heard offensive line coaches tell the OL to grasp the defensive lineman's chest plate of their shoulder pads. This would mean that the OL's fingers would be between the chest plate/jersey and DL's chest and the thumbs being visible on top of the jersey/chest plate.  

If this is done within the "guidelines" where there isn't a pulling of the shirt or outside of the elbows, is this legal and common practice?

Answer
Chris
"To grasp the defensive lineman's chest plate of their shoulder pads" is holding.   The rules provide Holding or illegal obstruction by a teammate of the ball carrier or passer is illegal.  The hand(s) and arm(s) shall not be used to grasp, pull or encircle in any way that illegally impedes or illegally obstructs an opponent.  Further, the hand(s) or arm(s) shall not be used to hook, clamp or otherwise illegally impede or illegally obstruct an opponent.  This is holding.  So the grasping of a defensive lineman's chest plate of their shoulder pads is illegal if it "impedes or obstructs the defender.  If the defender is not impeded by this act then there is is no foul.  It rests upon th judgment of the covering official whether there is a hold.  It is a common practice of lineman to grab or grasp an opponent {personally I wish that it not permitted, but since open hand blocking is permitted there will be a natural reaction to grab or grasp an opponent) but it does not rise to the level of a foul until there is an obstruction or impeding of the opponent caused by the grasp or grab.