I picked up the Fall 2007 Vogue Knitting and saw a pair of convertible mittens on page 36 and immediately thought- "I have to make a pair of those in crochet!". I like the speed of crochet and sure enough I was able to make a pair in a couple of days. I don't have a pattern because I didn't write anything down. They are done in single crochet using Red Heart Sport in two colors.

The "action shot", (I could only shoot one hand while holding the camera in the other).
I'm going to make another pair and correct some of the mistakes I made in this pair- this time I plan on writing the instructions down.
Hope you like...

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I made another pair (using worsted weight this time).

The instructions follow: (DISCLAIMER- I don't normally write down the instructions for what I make, I just crochet as I go. I hope you can follow this with a minimum of confusion).
CONVERTIBLE MITTENS
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Materials:
Size H crochet hook
Size F Crochet hook
Red Heart Super Saver 7 oz. Worsted weight acrylic
scrap yarn to use as stitch markers
Single Crochet = sc
Double Crochet = dc
Increase = inc
decrease = dec
yo = yarn over
The finished piece should be a closely stitched (a little stiff). Always use the back loop (loop closest to you). The piece is worked from the "wrong side" primarily in single crochet.
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With H hook, chain 30 and join. (This chain should be the size that fits around your arm where you want the mitten to begin).
From this point forward use the F hook.
SC in each chain stitch around (30 sc), join, ch 2.
DC in each sc (30 dc), join.
From this point the piece is done in a spiral, mark the beginning of each row.
Sc until the piece measures approximately 5 inches (or desired length to wrist). (Approximately 14 rows).
THUMB SHAPING:
SC the first stitch of the next row (marked stitch), 2 sc in next sc (1 inc). Sc in the next two stitches, 2 sc in next stitch (1 inc). Mark the next sc and sc the remaining stitches in the row.
(I like to mark the stitch before and the stitch after my increases).
Work even for the next row (sc in each sc).
Follow this pattern: Inc 1 sc after the first marker, sc in each stitch until one stitch before the next marker, inc 1 sc in that stitch, work a sc in each stitch for the remainder of the row.
Work the next row even (sc in each sc).
Follow this pattern until there are 14 sc between your markers. Work two even rows after that inc.
THUMB:
SC in each stitch after the first marker, sc the stitch before the second marker, chain 2. Join the chain to the stitch after the first marker with a sc, (mark this stitch as the beginning of the row). There should be 16 stitches in the join. Work even in sc on these 16 stitches until you have a length long enough for your thumb (approximately 6 rows).
Dec row: (bring up a loop in the next two stitches, yo and pull through three loops on hook- one dec made). Dec around until there are 8 stitches remaining. Cut a long tail and bring it through the last loop on the hook. Weave the tail into the remaining stitches. Pull tight to close the top of the thumb. Remove markers.
HAND OPENING:
Attach yarn two stitches before the chain 2, sc in each stitch around, there should be 28 sc stitches in the row. Work even in a spiral until the piece measures the length needed for your hand (approximately six rows). Join and chain 2. DC in each sc around, join and end.
MITTEN CAP (again using back loop):
Using F hook, chain 10. SC in second chain from hook, sc in next 7 chain stitches. In last chain stitch, 3 sc (mark second sc)- 1 inc, sc in each stitch on opposite side of chain.
3 sc in first sc stitch, (mark second sc)- 1 inc, sc in each stitch until marked stitch, 3 sc in this stitch (mark second sc).
When there are 3 inc's on each end, there should be 27 sc stitches in the row. Work even on these stitches until the mitten cap is long enough to fit over the fingers and over lap the mitten hand opening. (Approximately 9 rows), join, chain 2. DC in each sc around, join. SC in each dc, end.
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Make the second mitten exactly the same as the first. Attach the mitten cap to the back of the mitten.
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The first set of mittens was done in Sport weight, I used 40 stitches and an E hook for them. The striping was done by carrying the yarn behind the stitches.