When you want to keep track of counting a large number of objects, such as when you are stuck at a train stop watching the train cars roll by, you can use tally marks to count. Tally marks allow you to write down one line for each object you count.
When you reach the fifth object you are counting, draw a diagonal line across four tally marks. This indicates that you have counted five objects. This grouping of five helps you go back and easily add up all of the objects you counted. Counting by fives is easy to do in your mind when you add, and it is easy to count to five when you need to quickly write down tally marks.
Tally marks are useful when you have to count something fast. When you do not have time or the mental capacity to keep up with the rolling train cars going by quickly, you can simply make a mark on a paper and count them later. Tally marks are also useful in the classroom when a teacher needs a fast way to count behaviors of students to understand any patterns of behavior they have.
Tally marks being grouped in fives helps you visually see what a number is actually made up of. Instead of just seeing the number "5," you see that five marks were made to represent that symbol. This is useful in a classroom as well. You can demonstrate properties of addition and subtraction in a visual way to students. If you write "17" in tally marks, students can see that it is made up of three groups of five and two single units. Subtract 17 from 30, and you can show students the answer by crossing off 17 tally marks that are written for the number 30. The uncrossed-off tally marks added together gives the correct answer of 13.