13mm combination wrench. When working on your Volkswagen it a good idea to keep multiple wrenches for a few of the common fastener sizes. This 13mm chrome-vanadium combination wrench has both an open-end and a 12 point (triple-square) box-end. The 10mm size is common for many Volkswagen engine, electrical, and suspension components. So pick up a few of these so that you always have it on hand; the one you think you have is most likely lost in the abyss anyway.
The 13mm combination wrench and the 10mm wrench are used on many many bolts on VWs. Keep multiples around, just in case. Keep track of the work you do with your new tools with the portfolio shown below.