I was just looking up what the different ANSI lamp shape codes meant and came up with something weird. First of all I though "BT" was blown tubular, but it is actually bulged tubular. I feel like that rolls off the tongue less readily but whatever, not what I am talking about here. Here is my problem:

"E" stands for Ellipsoidal. "ED" stands for Ellipsoidal-Diffused. So the same as E, but diffused... That alone is weird to me, because we don't specify whether it is or isn't diffused in any other ANSI lamp shape code. There isn't any AD or BTD or PSD any of that crud.

Even more, E25 and ED18 are both similar elongated ellipsoidal shapes (the borderline-tubular shapes that I am not a huge fan of). Neither of these are diffused, despite the name of ED18 suggesting that it would be. That is because these rules are almost never ever followed for some reason. Barely anyone uses E to describe lamps, only ED, even when the lamps aren't diffused.

Why was a diffused version of the "E" code ever created, and why has E fallen out of favor for the incorrect use of ED to describe everything ellipsoidal, no matter it's translucency?

Where did you find it defined as Ellipsoidal-Diffused?  I've always heard that the D stood for Dimple or Dimpled.

Holy crud you're right. Embarrassing. Makes sense now.

BT is a little ambigous as i have seen at least 3 different lamp shapes referred to as "BT" -

 - Elliptical shape, superimposed over T neck and top - Many Iwasaki lamps

 - Cylindrical central section, little bulged, superimposed over T neck and top - Typical US 100/175W Mercury lamps

 - Tubular shape (with rounded top), with one small bulged part in the center - Osram HQI-T and their Chinese clones

@Ash
The elliptical shape superimposed on a T neck and top is what I traditionally think of with BT. Different sizes based on 1/8" as usual, commonly BT28, BT37, and BT56, but BT15 also exists and was used for some halogens, I am sure you have seen all of these.

That more cylindrical version common to 100/175W MV lamps is usually BT28 size, but I have also seen some rarer BT28s that look more normal, without sharp corners. Looking up "BT28 lamp" yields photos of both types of BT28 envelope. I personally am not huge fan of the less rare more cylindrical version that you are talking about, but it technically is still BT shape.

I have seen those tubular bulbs with only very small bulges with the HQI-BTs, I haven't seen any standard for that size. I haven't seen them referred as "BT##", just plain "BT". Unsure about the specifics there.

 I asked the "E" vs "ED" question back in 2019 and never got a definitive answer ! https://www.lighting-gallery.net/index.php?topic=7626.msg61829#msg61829  I own a 1970 GE 400w clear MV which is listed in the GE catalog as "E-37" and I also have several newer GE clear 400w MV which look completely identical but are listed as "ED-37" ! I also have a 1965 vintage 175w MV which looks completely identical to a newer 175w GE "ED-28" MV .

 Check out Lamptech , there's several GE catalogs on there . The "E" to "ED" change took place somewhere between 1993 and 1996 .

 Sylvania looks to have started using "ED" as early as 1982 .

@wide-lite 1000
Are your "E" shaped lamps dimpled like the "ED" ones?

 Yep ! https://www.lighting-gallery.net/gallery/displayimage.php?album=5200&pos=116&pid=163544

@wide-lite 1000
That is a very nice lamp there, and also an interesting use of the "E" designation for an "ED" shape bulb. Maybe naming conventions changed. IDK

Thanks !  I have NO idea either .

 I highly doubt we'll ever get a straight answer . 

E is elliptical without a dimple, and ED is elliptical with a dimple.

 Not true Dor !! Look at the photo I linked !! https://www.lighting-gallery.net/gallery/displayimage.php?pos=-163544  GE called that lamp an E-37 back in 1970 , It looks no different from the 2003 400w GE lamps I have which the 2003 catalog lists as ED-37 ! . As I stated before , The designation changed between 1993 and 1996 . The lamps all look identical !!