TRIAL COURT DISCRETION ARISES IN SUBSTANTIVE MATTERS SUCH AS DETERMINING THE CUSTODY OF CHILDREN AND THE DURATION OF SENTENCES OF OFFENDERS. IN SOME CASES, THE SOURCE OF COURT DISCRETION IS THE EXPRESS PROVISION OF A RULE, AND IN OTHER CASES, DISCRETION IS MANDATED BY STATUTE. DISCRETION ALSO CAN RESULT THROUGH A PROCEDURAL RULE IN WHICH APPELLATE DECISIONS HAVE INTERPOLATED THE IDEA OF DISCRETION. THERE ARE SITUATIONS IN WHICH NO FORMULARY RULE GOVERNS THE SITUATION AND YET JUDICIAL DECISIONS HAVE ESTABLISHED THAT DISCRETION IS LODGED IN THE TRIAL JUDGE. IT IS POSSIBLE TO SUPPORT THE EXISTENCE OF DISCRETION IN PROCEDURAL REVIEW BY REFERRING TO EXPRESS LANGUAGE IN STATUTES AND RULES, BY JUDICIAL INTERPRETATION OF RULES THAT ARE SILENT ON THE MATTER, AND BY DECISIONS IN COMMON LAW AREAS NOT SUBJECT TO FORMAL RULES. GRADE A LEVEL DISCRETION IS DESCRIBED; IT IS VIRTUALLY IMPERVIOUS TO APPELLATE OVERTURN AND IT IS UNREVIEWABLE AND UNREVERSIBLE. GRADE B LEVEL DISCRETION INVOLVES A SITUATION IN WHICH APPELLATE COURTS CAN DEFER TO THE TRIAL JUDGE'S DECISION, ORDINARILY WITH THE EXPLANATION THAT THE MATTER RESTS WITH THE LOWER COURT. GRADE C LEVEL DISCRETION, CAN BE EXERCISED BY THE TRIAL JUDGE TO DENY A NEW TRIAL OR CLAIM A MISTRIAL. GRADE D LEVEL DISCRETION IS DESCRIBED AS BEING EXTREMELY DILUTE. IT IS USED TO REFER TO AN AREA OF TRIAL COURT CHOICE THAT IS SHIELDED FROM THE KIND OF SEARCHING REVIEW THAT IS GIVEN TO A RULING ON A QUESTION OF LAW. WITH THIS KIND OF DISCRETION, THE TRIAL JUDGE'S FINDING MAY BE REVERSED FOR ABUSE, BUT IT IS SELDOM REVOKED. TWO REASONS FOR APPELLATE COURT DEFERENCE TO TRIAL COURT RULINGS ARE PROVIDED: (1) IT IS VIRTUALLY IMPOSSIBLE TO MONITOR THE COUNTLESS RULINGS THAT THE TRIAL JUDGE MAKES, AND (2) THE TRIAL JUDGE IS IN A POSITION TO WITNESS THE SITUATION. APPELLATE COURTS MUST BE DISCRIMINATING IN ACCORDING COMPLETE DEFERENCE TO THE TRIAL COURT'S DISCRETION. (DP)