From Wikipedia
"Nintendo employed several types of regional lockout, including both physical and hardware incompatibilities.
On a physical level, the cartridges are shaped differently for different regions. North American cartridges have a rectangular bottom with inset grooves matching protruding tabs in the console, while other regions' cartridges are narrower with a smooth curve on the front and no grooves. The physical incompatibility can be overcome with use of various adapters, or through modification of the console.Internally, a regional lockout chip (CIC) within the console and in each cartridge prevents PAL region games from being played on Japanese or North American consoles and vice versa. The Japanese and North American machines have the same region chip. The console CIC releases the reset signal to the rest of the system only after completing a handshake with the chip in the cartridge. This can be overcome through the use of adapters, typically by inserting the imported cartridge in one slot and a cartridge with the correct region chip in a second slot. Alternatively, disconnecting one pin of the console's lockout chip will prevent it from locking the console; hardware in later games can detect this situation, so it later became common to install a switch to reconnect the lockout chip as needed. PAL consoles face another incompatibility when playing out-of-region cartridges: the NTSC video standard specifies video at 60 Hz while PAL operates at 50 Hz, resulting in approximately 16.7% slower gameplay. Additionally, PAL's higher resolution results in letterboxing of the output image. Some commercial PAL region releases exhibit this same problem and therefore can be played in NTSC systems without issue, while others will face a 20% speedup if played in an NTSC console. To mostly correct this issue, a switch can be added to place the SNES PPU into a 60 Hz mode supported by most PAL televisions. Later games will detect this setting and refuse to run, requiring the switch to be thrown only after the check completes."
