AN SF340 CROSSES RWY 4L AT BOS WITHOUT CLRNC WHILE A C402 IS ON A 2 MI FINAL.

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IN BOSTON THE SEQUENCE OF ATC ALWAYS IS CLRNC TO TAXI; THEY TELL YOU TO MONITOR GND. GND ISSUES TAXI INSTRUCTIONS. FROM GND YOU MONITOR ONE OR BOTH TWR FREQS. TAXI INSTRUCTIONS TO RWY 9 ALWAYS INCLUDE HOLD SHORT OF RWY 4L. THE GND CTLR ADVISED 'YOUR SEQUENCE IS TO FOLLOW ACFT XY; MONITOR THE TWR ON 128.8.' AS WE REACHED TXWY S; I SLOWED TO A STOP. THE FO SAID 'YOU'RE CLR R' MEANING RWY 4L WAS CLR TO CROSS. I QUESTIONED HIM 'OH; WE'RE CLRED TO CROSS?' 'YES' HE SAID. A C402 WAS ON A 2 - 2 1/2 MI FINAL TO RWY 4L; PLENTY OF TIME TO CROSS. WHEN WE WERE 2/3 OF THE WAY ACROSS RWY 4L; THE 128.8 TWR CTLR ASKED US IF WE WERE ON FREQ. THE FO ANSWERED; 'YES.' HE SAID; 'WELL; YOU WEREN'T CLRED TO CROSS RWY 4L; BUT SINCE YOU HAVE; MONITOR THE TWR 119.1.' THE FO APOLOGIZED AND EXPLAINED HE MISUNDERSTOOD THE GND CTLR THINKING WE WERE CLRED TO CROSS BEHIND ACR XY. THAT; HOWEVER; WAS ONLY OUR SEQUENCE; NOT CLRNC ACROSS. I WAS WRONG BECAUSE THE SEQUENCE OF COM IN BOSTON IS THE SAME ALL OF THE TIME (CLRNC; GND; 128.8; 119.1 FOR RWY 9); AND I DIDN'T MAKE THE FO CONFIRM WE WERE CLRED TO CROSS BECAUSE I HADN'T HEARD IT. IT IS ALWAYS 128.8 WHO CLRS YOU ACROSS RWY 4L; NOT GND CTL.