I guess I'll add what I know about this subject. If we are talking about creating a Bb13th chord, then my understanding of chord structure is that the dominant 7th must be stated somewhere when playing the 13th note, otherwise what you are playing is a Bb6th. Normally any of the 9ths, 11ths, and 13ths should have the dominate 7th heard somewhere to "legitimze" the chord, although as a guitar player I have played a simple 6th chord and gotten away with it. When creating some of those chords that require 10 fingers on the fretting hand, I build from a "shell" chord (root, 3rd,7th) many times leaving out the root if there is another instrument (bass) playing that note. A Bb7/13 chord although unusually noted, would as others have stated be a Bb dominant 7th with a G in the bass. Hope this helps. DennisD
PS to John C thanks for noticing my 100th post

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