Kaeote, you asked how to write a flat in BIAB. It's lowercase B on the keyboard.

All the rest has been explained well, but I'll just add a quick tip for anyone reading this thread: When you talk about intervals above an octave, just subtract 7 to find the pitch. A 9th is the same as a 2, an 11th a 4, and a 13th is the same pitch as a 6.

From your last sentence, it does appear that you are looking for a Bb7 add 13. As mentioned, BIAB does not support this. If indeed as Mario noticed, you write a slash root format, you will get the 13th in the bass, as in Bb13/G.

Again for anyone who might not know, a 13th chord has a distinctive sound because it has both the 6th and 7th, which are a half-step apart (although voicing the 6th as a 13th puts it up a major seventh over the dominant seventh, so it does not sound as dissonant). And I concur that a 13th implies not only that the dominant 7th is present, but also the 9th and often the 11th.

BIAB 2025 Win Audiophile. Software: Studio One 7 Pro, Swam horns, Acoustica-7, Notion 6, Song Master Pro, Win 11 Home. Hardware: Intel i9, 32 Gb; Roland Integra-7, Presonus 192 & Faderport 8, Royer 121, Adam Sub8 & Neumann 120 monitors.