Beginning in the 2040s, with the advent of fusion power, Terra began to place space settlements at the Terra/Luna Trojan Points. The first of these were named L4 and L5, respectively, after the Lagrange Points they occupied.
As time passed, more settlements were constructed. By 2068
CE, for example, it was estimated that over 8,000 settlements
were available for habitation. Roughly a tenth of these were in
Earthspace, five percent in Marsspace, and the rest in the asteroid
belt or further into the outer solar system.
Additional settlements in the Trojan Points were named L6, L7, L8, etc. Not just numbers were used: settlements were named LA, LB, LC, etc. -- and when letters gave out, even punctuation symbols were pressed into use. (One of the largest and most influential of the early settlements was the Schmidt Foundation's L%.) After this, settlements continued to be named with the initial L, leading to a large number of ludicrous results (including Settlement #5295, Ludicrous Land) as well as vast confusion.
Fortunately, with the advent of interstellar travel many settlements were converted to colonization vessels, and in the process given new names.