B737 CAPT DSNDED FROM THE SPECIFIED STAR TRANSITION ALT PRIOR TO BEING CLRED TO DO SO BY THE APCH CTLR DUE TO HABIT OF MAKING THE SAME ARR ON MANY OCCASIONS.

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IN THE DSCNT TO SEA; WE WERE ON THE OLYMPIA ARR TAR; WHICH HAS 2 DIFFERENT SETS OF ALT RESTRS; BASED ON THE LNDG RWY IN USE AT SEA. THIS DAY THE LNDG DIRECTION WAS N; SO THE XING ALT AT OLM VOR WAS 12000 FT. WHEN THE LNDG DIRECTION IS S RATHER THAN N; THE ALT FOR XING OLM IS 16000 FT; AND ONCE THAT HAS BEEN MET; YOU DSND ON YOUR OWN TO 12000 FT. BUT WHEN LNDG N AS TODAY; YOU MUST MAINTAIN 12000 FT AND WAIT FOR THE CTLR TO DSND YOU DOWN TO 10000 FT. I WAS FLYING WHILE MY FO WAS ENGAGED IN COMS WITH THE COMPANY OPS PEOPLE; AND WHILE WE WERE DOING THAT; WE REACHED 12000 FT OVER THE OLM VOR. SINCE WE ROUTINELY DIAL IN THE NEXT LOWER ALT WHEN LNDG S; AND BECAUSE IN THE PAST SEVERAL WKS I HAD FLOWN THE ARR TO THE S EACH TIME; I MISTAKENLY DSNDED BELOW 12000 FT TO 10000 FT; SOMETHING YOU CANNOT DO WITHOUT CTLR CLRNC WHEN; AS TODAY; THE LNDG DIRECTION IS N. THE FO CONCLUDED HIS RADIO DUTIES AND NOTED I WAS ALMOST LEVEL AT 10000 FT. HE CAUGHT THE ERROR; BUT RIGHT AT THE TIME I WAS STABILIZING THE LEVELOFF; AND RIGHT WHEN THE CTR CTLR HANDED US OFF TO SEA APCH CTL -- TOO LATE TO CLB BACK. IN THE INITIAL CONTACT WITH APCH; WE GAVE OUR CALL SIGN AND 'HAVE ATIS INFO.' THE CTLR ISSUED A CLRNC TO DSND AND MAINTAIN 7000 FT. FROM THAT POINT ON TO LNDG; ALL NORMAL CTLR CLRNCS WERE GIVEN; AND NO MENTION WAS MADE TO US OF OUR (MY) ERROR! THE CAUSE WAS SURELY COMPLACENCY BROUGHT ON BY OVER FAMILIARITY AND THE HABITS FORMED BY DOING THE ARR THE OTHER WAY SO MANY TIMES IN A ROW. I HAD THE APCH PLATE OUT; AND IN FRONT OF ME; BUT DIDN'T REFER TO IT; NO DOUBT BECAUSE I WAS 'SO SURE' OF HOW TO CONDUCT THE ARR. BIG MISTAKE -- AND AN EMBARRASSING ONE; TOO! I AM USED TO TAKING EXTRA CARE WITH MATTERS OF CLRNCS; READBACKS; AND ALT AWARENESS OVERALL; AND TODAY I COMPLETELY DISRESPECTED MY OWN STRINGENT WAY OF DOING THINGS AT A TIME WHEN MY FO WAS OTHERWISE ENGAGED. MURPHY'S LAW WAS OPERATING AT ITS BEST.