Someone from the plumbing trades comes in and asks for a “wye”. Someone from a steamfitter background calls looking for a “45-degree lateral”. Which fitting weighs more? Trick question! They are the same fitting.
Decades ago, our company started restricting the conversation to two terms to cull the confusion. A straight run with a branch to one side is a LATERAL. A fitting with a trunk coming up that splits to a 45º branch left and right is called a TRUE WYE, because it resembles a “capital letter Y” as we learned to draw it in grade school.
Plumbers work extensively with PVC and ABS. If you look in the catalogs from Charlotte®, Genova®, or various other manufacturers in the plastics industry, what a steamfitter will refer to as a lateral, is referred to in those catalogs as a wye.
Not confused enough? The plumbing trade calls a 45-degree elbow a “1/8th bend”; because it turns 1/8th of a 360-degree circle. So when a customer calls for a “combination wye & 1/8th bend” (a.k.a. “combo”) They are referring to what we call a lateral-tee. A lateral-tee is simply a lateral with a 45-deg elbow welded to the branch directed perpendicular to the run.