X.690 is an ITU-T standard specifying several ASN.1 encoding formats:
The Basic Encoding Rules (BER) were the original rules laid out by the ASN.1 standard for encoding data into a binary format. The rules, collectively referred to as a transfer syntax in ASN.1 parlance, specify the exact octets (8-bit bytes) used to encode data.
X.680 defines a syntax for declaring data types, for example: booleans, numbers, strings, and compound structures. Each type definition also includes an identifying number. X.680 defines several primitive data types, for example: BooleanType, IntegerType, OctetStringType. (ASN.1 also provides for constructed types built from other types.) Types are associated with a class. For example, the primitive types are part of the universal class. The three other classes (application, private, and context-specific) are essentially different scopes to support customization for specific applications. Combined, the class and type form a tag, which therefore corresponds to a unique data definition. X.690 includes rules for encoding those tags, data values (content), and the lengths of that encoded data.
BER, along with two subsets of BER (the Canonical Encoding Rules and the Distinguished Encoding Rules), are defined by the ITU-T's X.690 standards document, which is part of the ASN.1 document series.