When a fitting has a socket on each approach in this case a junction. You look at it you will notice that the two inlets, the pipe sits with the bell end facing up stream. The outlet, the bell end is facing down stream.
Now do you see what Bob meant.
Sorry Bob, your from FNQ not the sunshine coast
All advice is given with-out seeing the job, and hence all advice MUST be taken as advice with limited knowledge on the exact situation. NO responsibility can or will be taken. And yes, I am a licensed Plumber and Drainer with my own business in Brisbane Australia