They are in effect a trench firing step hardened to protect against small-arms fire and grenades and raised to improve the field of fire.
The concrete nature of pillboxes means that they are a feature of prepared positions. They were probably first used in the Hindenburg Line. This is likely to have been the time when they acquired their incongruous English name. The Oxford English Dictionary's earliest record of the use of the word pillbox in connection with a defensive post is from 13 September 1917, after the German withdrawal onto the Hindenburg Line.
Pillboxes are often camouflaged in order to conceal their location and to maximize the element of surprise. They may be part of a trench system, form an interlocking line of defence with other pillboxes by providing covering fire to each other (defence in depth), or they may be placed to guard strategic structures such as bridges and jetties.
The French Maginot line built between the world wars consisted of a massive bunker and tunnel complex, but as most of it was below ground little could be seen from the ground level. The exception were the concrete blockhouses and pillboxes which were placed above ground to allow the garrison of the Maginot line to engage an attacking enemy.
About 28,000 pillboxes and other hardened field fortifications were constructed in England in 1940 as part of the British anti-invasion preparations of World War II. About 6,500 of these structures still survive.
Pillboxes for the Czechoslovak border fortifications were built before the Second World War in Czechoslovakia in defence against the German invasion of Czechoslovakia. None of these were actually used against its intended enemy, since the German military met no resistance when invading the country because it was effectively forced to capitulate as a result of Allies annexing the country's border areas and handing them to Germany, but some were used against the liberating Russian armies. The Japanese also made use of pillboxes in their fortifications of Iwo Jima.