The question about guides in Illustrator
The question was trigged by the screenshot you see here. The screenshot shows diagonal guides in Illustrator. When Ahmed Chergaoui saw this he asked:
"(maybe) a stupid question if you mind : are these guides? Is it possible to have diagonal guides in Illustrator? :)"
You can turn a line, a path or any kind of shape into a guide in Illustrator, simply by selecting it and hitting command/control + 5. I took the above screenshot when I was working on this logo design proposal. I first drew the skewed rectangle shapes and then I added the guides afterwards to correct the depth a bit. I simply drew these lines using the Line tool and then used the shortcut. You can also access this via View > Guides > Make Guides. It's been there for ages, but it seems a lot of people don't know this.