In the past, they simply had the S (Sonderklasse) and a second car with no actual name (which became the E-Class). Then other letters were added, according to engine or chassis.

The “E” originally stood for “Einspritzung” (=injection). So the SEL was “Sonderklasse Einspritzung Lang”.
Since the smaller car only had the E for Einspritzung, it later became known as the E-Class.

With sportscars, it was different. SL originally stood for “Sport Leicht” (the irony), although some claim that it stands for “Super Leicht”, but I guess they’re confusing it with the Italian “Superleggara”. Anyway, so what does SLS stand for? “Super Leicht Sport” would be possible, but it goes against the Mercedes naming convention. But that doesn’t seem to concern anyone as of late…who knows what the A-Class stands for? Or the B-Class? The R-Class? The M-Class?

Someone should ask Mercedes…I know some of their older meanings:

S = Sport / Sonderklasse
L = Leicht / Lang
C = Coupé
T = Transport (station wagon)
D = Diesel
E = Einspritzung
R = Rennsport (racing)
G = Gelände
K = Kurz / Kompressor

But ever since Mercedes changed the names of their cars during the 1990s by putting a letter up front, they seem to have lost their meaning…at least partially.