Consider a scenario where our electronics project prototyping is done and we are getting the expected results. Now we want to make a production device out of it. We need to remove the breadboard and jumper wires and create printed circuit board (PCB) with an equivalent circuit. This expansion PCB will connect to BeagleBone via the P8 and P9 expansion headers. It will work as a plugin daughterboard for BeagleBone. This type of expansion board is called a cape. A cape for BeagleBone is similar to what a shield is for Arduino. This naming suits it because it looks like a cape around the Ethernet port when plugged in. BeagleBone comes with less peripherals on the board. Capes extend the capabilities of BeagleBone to have extra peripherals such as an LCD, motor driver, or Real-time Clock (RTC). Some capes transform BeagleBone into totally different products like Replicape converts BeagleBone into open source 3D printer. Popular capes are listed here:

Four capes can be stacked on to BeagleBone. The BeagleBone System Reference Manual (SRM) has a full chapter dedicated to capes. It provides you with many guidelines when creating a new cape. Capes needs to have 32 Kbyte I2C EEPROM connected to P9_19 and P9_20 I2C pins. It should contain board information such as the board name, serial number, revision, and so on. This EEPROM also has the information to autoconfigure the required expansion header pins. I2Caddress 0x54 to 0x57 are reserved for a BeagleBone cape. If there are two capes stacked on BeagleBone that have the same address, there will be address conflict. So, I2C addresses should be configurable via a jumper or dips switch. Then, by changing jumper/switch, you will force cape to use another address.

BeagleBone Black shares some expansion pins with the HDMI and emmc chip. BeagleBone Green shares a few pins with the emmc chip. Any cape dealing with these shared pins cannot be used with these boards. Though HDMI and emmc can be disabled, the hardware chips are still present on the board and can create problems. Here is the Wiki page that has a chart of capes of BeagleBone Black:

http://elinux.org/Beagleboard:BeagleBone_Black_Capes

Software part that deals with addition/deletion/controlling capes is called Cape Manager. Cape manager is part of BeagleBone kernel. It can load device tree runtime which is called device tree overlay. Cape manager checks for cape at the time of booting. You can see that in early boot logs:

bone-capemgr bone_capemgr.9: slot #0: No cape found

On BeagleBone Black, emmc storage and HDMI appears as virtual capes. You can also enable/disable cape by kernel command line options. For more information visit http://elinux.org/Capemgr