Slimline cards have become very popular recently and with good reason. They're super easy to make, appropriate for any occasion and can be altered for most card-making styles. There are no hard and fast rules for making slimline cards, just a few things to consider regarding envelopes, decorations and size ratios.
I find it easiest to think about my envelope before deciding on the overall size of my slimline card. Most countries will have a business size envelope that corresponds with the standard paper size for letter-writing. In Australia, our standard paper size is A4 and our business envelopes are DL.
If you don't like making your own envelopes, it's a great idea to size your slimline card to fit one of the standard sizes. For A4, you'll need to remove about 1/3 of the length, which is approximately 10cm.
I find business envelopes, and therefore business-sized cards, a little large for my liking and prefer to have a smaller card and make my own envelope. A good ratio for slimline cards is 1:2, which means the height is half the measurement of the length. To make that super simple, just cut a square of cardstock and fold it in half! Mine is 6" square, which creates a folded card measuring 3" x 6".
To decorate, I have used one image layered multiple times across a sheet of cardstock. You could also use a very tall or long image, or a series of die cuts, multiple frames, a big ribbon bow... just about anything really.
Trim the image panel to fit the front of your slimline card and add any extra embellishments to finish. I secured some scrap pieces of chipboard to the back of the panel for a little more dimension and weight, then added a sheer ribbon and some glittery stickles.
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