What is it ?, I hear you cry plaintively ..
Crochet A–Long. And, as you may work out eventually, there are aso KALs. :D
Someone online – it can be ANYONE ! – announces a CAL on her blog. She describes her plan for whatever it is; she usually talks about yarn and hooks and gauge and stuff; she might provide suggestions for specific yarns or colours. The object she has planned – an afghan, a shawl, a blanket, etc. (it has to be something fairly large, to occupy a number of posts) – is divided into [X] periods and the starting date provided. Almost every CAL ever dreamed up is then packaged in a Facebook site, where everyone (except me) can join and post images of how they’re going.
You have KALs like this:

and CALs like this – motifs are always popular:

and even CALs like this:

Many of them are shown in advance, so crocheters know what the completed thing looks like. But of recent years there’s been a trend to have them as ‘mystery’ projects: you only know which bit you have to do in the next period (however long it be).
Mine is a MYSTERY CAL: until it began, last Wednesday, no-one other than the organising blogger had the faintest idea of anything about it – other than it was to be in Tunisian crochet. That’s why I signed up for it: I wanted to learn more Tunisian stitches.
First 30.5cm square and I couldn’t even begin. Arunima, whose blog is the originating one, had chosen for the first of the crochet experts she’d arranged to provide the 24 designed squares, one per fortnight, the marvellous Rachel Henri – whose gallery of beautiful Tunisian crochet designs includes one that I HAVE ACTUALLY MADE !!! Rachel, in turn, had chosen a square that she’s constructed in the round. It requires a double-ended hook. A 25cm 8mm one. I do not possess one of these. I do have two cro-hooks – double-ended and enormous, which I can’t use for this design because it involves twisted stitches that cause the cro-hook to make huge bruises on my hand between thumb and forefinger. *
I whinged on Arunima’s site, and she says that she managed it thus:

Anyone who can explain HOW she does this will win a prize. Including Arunima, of course. Here’s an image of interchangeable hooks – I have this very set:

You take a length of ‘cable’ (the red things on the right), affix your required size hook (the coloured things on the left) and add the end other end of the cable one of the black stoppers (below the hooks). I think Arunima is telling me that she starts off each pattern row with this setup – which includes the large red 8mm hook – then when she’s reached the end of a side of the square, she removed the cable/stopper and .. does WHAT ??: takes each stitch off from the non-working end of the hook and puts it onto the 6mm hook ? I tried that and ended screaming .. you would have to know this square’s pattern to understand ..
So this is as far as I’ve got .. and I go NO FURTHER:

That blue crochet hook (I managed to get this on-line from a totally reliable supplier, who sent it, as required, by Express Post and it arrived the next day !) is only 15cm long, and I cannot continue to build the square. Oh, BTW: nearly all Tunisian stitches curl frightfully, because they are different .. ahh .. tightnesses between front and back of stitch; they need robust and vigorous blocking.
I shall not crochet another stitch until I have a 25cm or so 8mm double-ended crochet hook in my hands. As the CAL goes for virtually the entire year and I can make this square whenever, it takes a back seat till then. Rachel has done her best to recommend sites for purchase; but the Aussie one’s no longer active, one’s Amazon UK (shipping prices make my eyes water) and one’s French and not shipping to Oz. Sighh ..
* Here’s where I tell those who don’t already know how I crochet. It’s unspeakable. I hold the yarn in my right hand, and I use the crochet hook like a knitting needle – pushing it R/L into stitches, rolling the yarn over and pulling the hook back. I’ll try to video myself doing it, one day ..