As far as I can tell, after having read Martin Richards's BCPL book and the BCPL reference manual, BCPL allowed only $( $) as section brackets. Looking at the

As far as I can tell, after having read Martin Richards's BCPL book and the BCPL reference manual, BCPL allowed only $( $) as section brackets. Looking at the current BCPL compiler code, it seems that it does now allow { } for section brackets, but the comments in the compiler suggest that it wasn't added until 1995. BCPL's section brackets were neat in that you could add a label to them and they would be parsed much like HTML tags. I think it's likely that B introduced the curly brackets in lieu of BCPL-style section brackets and omitted tagged brackets altogether, then B begat C, etc.