Crochet abbreviations

Most used ones:

ch - chain
sl st - slip stitch
dc - double crochet (US sc)
tr - treble (US dc)
htr - half treble (US hdc - half double crochet)
dtr - double treble (US trc - triple crochet)

Others:

alt - alternate

beg - beginning
blo - back loops only

cl - cluster

dc2tog - double crochet together
dec - decrease
dtr2tog - double treble 2 sts together
dtr3tog - double treble 3 sts togehter

rep - repeat
RS - right side

tr2tog - treble 2 together
trtr - triple treble

WS - wrong side
yo - yarn over hook
yrh - yarn round hook