Question: When I use `svn diff` or `git diff` it shows lines like:

```
@@ -1,5 +1,9 @@

```

What do they mean?

Answer: 
**Simple example analysis**

The format is basically the same the `diff -u` unified diff.

For instance:

```
diff -u <(seq 16) <(seq 16 | grep -Ev '^(2|3|14|15)$')

```

Here we removed lines 2, 3, 14 and 15. Output:

```
@@ -1,6 +1,4 @@
 1
-2
-3
 4
 5
 6
@@ -11,6 +9,4 @@
 11
 12
 13
-14
-15
 16

```

`@@ -1,6 +1,4 @@` means:

* `-1,6` means that this piece of the first file starts at line 1 and shows a total of 6 lines. Therefore it shows lines 1 to 6.

```
1
2
3
4
5
6

```

`-` means "old", as we usually invoke it as `diff -u old new`.
* `+1,4` means that this piece of the second file starts at line 1 and shows a total of 4 lines. Therefore it shows lines 1 to 4.

`+` means "new".

We only have 4 lines instead of 6 because 2 lines were removed! The new hunk is just:

```
1
4
5
6

```

`@@ -11,6 +9,4 @@` for the second hunk is analogous:

* on the old file, we have 6 lines, starting at line 11 of the old file:

```
11
12
13
14
15
16

```
* on the new file, we have 4 lines, starting at line 9 of the new file:

```
11
12
13
16

```

Note that line `11` is the 9th line of the new file because we have already removed 2 lines on the previous hunk: 2 and 3.

**Hunk header**

Depending on your git version and configuration, you can also get a code line next to the `@@` line, e.g. the `func1() {` in:

```
@@ -4,7 +4,6 @@ func1() {

```

This can also be obtained with the `-p` flag of plain `diff`.

Example: old file:

```
func1() {
    1;
    2;
    3;
    4;
    5;
    6;
    7;
    8;
    9;
}

```

If we remove line `6`, the diff shows:

```
@@ -4,7 +4,6 @@ func1() {
     3;
     4;
     5;
-    6;
     7;
     8;
     9;

```

Note that this is not the correct line for `func1`: it skipped lines `1` and `2`.

This awesome feature often tells exactly to which function or class each hunk belongs, which is very useful to interpret the diff.

How the algorithm to choose the header works exactly is discussed at: [Where does the excerpt in the git diff hunk header come from?](https://stackoverflow.com/questions/28111035/where-does-the-excerpt-in-the-git-diff-hunk-header-come-from)
These describe the lines affected by the diff hunk. In your case, it means the hunk affects 5 lines starting from line 1, resulting in a replacement starting at line 1 which is 9 lines long.

Note that this is the format used by the unified diff format. The "classical" diff format uses a different model (but who uses classical diff these days?).