A letter is any of the 26 lowercase or 26 uppercase letters in the basic execution character set.
The decimal-point character is the (single-byte) character used by functions that convert between a (single-byte) character sequence and a value of one of the floating-point types. It is used in the character sequence to denote the beginning of a fractional part. It is represented in Clauses [language.support] through [thread] and Annex [depr] by a period, '.', which is also its value in the "C" locale, but may change during program execution by a call to setlocale(int, const char*),162 or by a change to a locale object, as described in Clauses [locales] and [input.output].
A character sequence is an array object A that can be declared as T A[N], where T is any of the types char, unsigned char, or signed char ([basic.fundamental]), optionally qualified by any combination of const or volatile. The initial elements of the array have defined contents up to and including an element determined by some predicate. A character sequence can be designated by a pointer value S that points to its first element.
A null-terminated byte string, or ntbs, is a character sequence whose highest-addressed element with defined content has the value zero (the terminating null character); no other element in the sequence has the value zero.163
Many of the objects manipulated by function signatures declared in <cstring> are character sequences or ntbss. The size of some of these character sequences is limited by a length value, maintained separately from the character sequence.
A string literal, such as "abc", is a static ntbs.
An ntbs that contains characters only from the basic execution character set is also an ntmbs. Each multibyte character then consists of a single byte.