Cards are great. I love the sound they make as you riffle-shuffle them, I love dealing them out, and I love the tension of the very simple mechanic of turning over the top card of the deck. Every new card is rife with ever-present possibility, and promises joy, delight or despair.
If you have a favourite game that involves cards you'll be shuffling a lot and sliding across a table, it's worth protecting your investment with a set of card sleeves. Which is harder than you'd expect it to be.
Incidentally: here's a neat trick you can try on your friends. Proper playing card stock (as opposed to regular cardboard) is built with natural springiness. You should be able to bend a card, end-to-end, and the card will curve neatly in a circle. A regular bit of cardboard will bend and be forever creased.
Most cards from most professionally published board games are made from proper card stock, and you can test. This is an excellent way to freak out your fastidious friend.
But I digress.
The point is: you want to protect your precious cards from your favourite game from maniacs like me who bend and riffle-shuffle your cards. The best way to do this is to protect your cards with plastic sleeves.
Now if you come from a background in TCGs (trading-card games, such as Magic: The Gathering, Android: Netrunner, Yu-Gi-Oh! etc) you might assume that this would be as simple as deciding on what colour backing you want on your cards. Nothing could be further from the truth.
There are standards in the world of board games. The problem is that there are just so many standards. Here is one card sleeve size finder from Mayday Games:
Note that this is not an exhaustive list! This is a list of the size of the most common sleeves, not the size of the cards that are printed. Not unexpectedly, differences in printing exist. Printers in Europe and printers in China might differ subtly despite aiming for the same standard.
For instance, I measured cards for two board games that are both nominally the same size as Magic: The Gathering. One measured 88 x 63mm. The other measured 88 x 63.5mm. This means the former fit in KMC perfect-fit sleeves, and the other... won't.
So the task is more complicated than simply locating your friendly local games store. You should do some research and measuring to avoid staring at a wall of sleeves, jaw agape, without enough information to decide.
How do you know what size sleeves to buy for your cards?
Recently, I came across a secret. Just as BoardGameGeek is the definitive list of all board games ever made, there's a list on BoardGameGeek containing all the sleeving information you'll ever need for almost every game that's worthy of being sleeved: Card Sleeve Sizes for Games. At last count, it had detailed information for 3668 of the most popular and highly-rated board games.
It has information about most major sleeve manufacturers (Ultra-Pro, Mayday, and the like) as well as some of the more recent entrants into the field (such as the recent Paladin range launched by NSKN on Kickstarter). It contains information about sleeve thickness (measured microns: 40 microns is thin, 100+ micros is a thick, premium sleeve). It lists all known sizes of all major sleeve manufacturers that are known to fit.
And, the BGG community being what it is, in the comments for each game are additional comments and notes. For instance, in the comments, I learned that one of the expansions for Innovation (one of my favourite games) is bigger than the base game, and so you need different sleeves to accommodate this.
So hit up the list, find your game, and choose your sleeves with confidence. From there, all you have to do is visit your favourite online retailer or friendly local game store.