Frenched headlights became an American styling fad in the 1950s. The practice was to extend the front fender over or around the headlight to lengthen the car body both physically and visually. The latter had to do with the longest body line seemingly set near the car's "shoulder height" rather than having the longest massing down at bumper level. (Actually, the bumpers were at the extremes of the cars, but stylists diverted attention to the highlight line just mentioned.)
The fashion aspect was dying out by the early 1960s, though frenched production cars sometimes emerged until as late as the 1970s, when the need for aerodynamic efficiency was forced on the industry due to high fuel prices and government dictates. Frenched headlights are not normally compatible with airflow smoothing.
Headlight frenching on 1949-51 Lincolns was unintentional, the story goes. It seems that the plan was to have the headlights hidden behind doors (a feature on 1942 DeSotos and a common practice in the 1960s and later). But technical issues prevented Lincoln from incorporating this feature, so sunken headlight bezels were substituted.
This 1951 Pontiac illustrates normal headlight appearance before the frenching fashion got underway.
Frenching came on strong for the redesigned 1952 Mercurys and Fords.
At the time, a few other brands also began to "french," but much more timidly. You have to look closely at the Cadillac image above (try clicking on it to enlarge) to see the hint of frenching on the upper part of the chromed bezel.
Frenching soon became bolder for Ford-Mercury competitors.
Here is an example of leaning the frenched bezel forward to extend the length of the car at fender-top level.
Examples of late-1950s frenching.
The re-styled 1971 Camaro featured traditional frenching somewhat like on the '55 Mercury shown above, but more subdued with the largest extension below the headlamps, rather than above them. The '71 body remained in production through the early oil-crisis years, so stylists reworked the frenching to make the front of the car seem more aerodynamic, even though actual improvement was probably minimal.