A reference is used in git to specify branches and tags. A branch head is stored under $GIT_DIR/refs/heads directory, and a tag is stored under $GIT_DIR/refs/tags directory. git imposes the following rules on how refs are named:

1.

It can include slash / for hierarchical (directory) grouping, but no slash-separated component can begin with a dot

2.

It cannot have two consecutive dots anywhere;

3.

It cannot have ASCII control character (i.e. bytes whose values are lower than \040, or \177 DEL), space, tilde ~, caret ^, colon :, question-mark ?, asterisk *, or open bracket [ anywhere;

4.

It cannot end with a slash /. These rules makes it easy for shell script based tools to parse refnames, pathname expansion by the shell when a refname is used unquoted (by mistake), and also avoids ambiguities in certain refname expressions (see git-rev-parse(1)). Namely:

1.

double-dot are often used as in ref1..ref2, and in some context this notation means ^ref1 ref2 (i.e. not in ref1 and in ref2).

2.

tilde ~ and caret ^ are used to introduce postfix nth parent and peel onion operation.

3.

colon : is used as in srcref:dstref to mean "use srcref's value and store it in dstref" in fetch and push operations. It may also be used to select a specific object such as with git-cat-file(1) "git-cat-file blob v1.3.3:refs.c".