Brass tubular products such as; traps, extension tubes, wall arms, tailpieces, etc. come in various wall thickness which is identified by gauge thickness. VMS stocks 17 gauge (thicker walls) used in commercial and industrial applications and 20 and 22 gauge (thinner walls) that are typically used in residential and multi-unit housing.
VMS stocks tubular diameters of 1-1/4, 1-1/2 and 2” as well as, nuts and slip joint washers.  1-1/4” tubular is generally found in lavatory sink installations where there’s less drain flow.  1-1/2” tubular is generally found in kitchen sinks and 2” is commonly used in commercial kitchen scullery sink drains.
Slip joint is the most common connection to use when joining tubular products.  This is where a portion of the tubular opening is expanded in order to accept the non-expanded tubular part. This connection utilizes a slip nut and a slip washer. There is also a direct connect method where the ends of the pipe are bent outwards to form a flange or tee shape.  In this application you use a flat tailpiece washer and slip nut, which tightens over a kitchen sink basket strainer.  And of course some are threaded connections.