In case you need a refresher course, to subitize (pronounced soo-bi-tize) means to instantly recognize a small quantity of objects without counting. When you hold up four fingers and a student says 4 without counting, they are subitizing. Once students can recognize small numbers, they are able to combine those numbers to make bigger numbers. For example, in the picture at the top of the page, a child might see the 4 dots as 3 and 1 more, or 4. Students should practice subitizing with a variety of objects and arrangements. Five- and ten-frames are great for subitizing because of their anchors to 5 and 10.
I just finished up a set of flashcards showing the numbers from 1 to 20 with ten-frame representation, numeral, and word form. Click here to check them out at my TpT store!