Octal

Sometimes documentation describes bit patterns using groups of three. Three-bit groups are named using the first eight pattern names of hexadecimal. This method is called octal notation. A bit pattern can be named using hexadecimal names, octal names, or several other notations.

01101010 = 01 101 010 =  152 (octal)
01101010 = 0110 1010  = 0x6A (hex)

Octal is awkward to use with 8-bit bytes. Bytes don't evenly split into octal pattern names. But you should know about it. Historically, some computer documentation used octal pattern names. Also, in several programming languages (C and Java among them) an octal literal is indicated by a leading zero:

0152    (octal)    = 001 101 010 
0x152   (hex)      = 0001 0101 0010 
152     (decimal)  = 1001 1000 

When the number of bits is not a multiple of three it is conventional to add zero bits to the left, and then to name the pattern as usual. This happens frequently because computer memory is organized into 8-bit bytes, which do not divide evenly into groups of three.

I have lost an unfortunate number of hours with buggy programs, only to discover a constant buried deep in the code that started with a "0" when it should not have.