A number line can be a line of any length that lists positive and sometimes negative numbers. Such lines are commonly used for young children learning arithmetic functions like addition and subtraction. They also come into use when children are taught about the “in between numbers” like fractions. Being able to plot fractions or decimals on a line is a helpful way of understanding the real size of a fraction and its comparative nature to other fractions.
In the standard horizontal number line, the zero is called the origin. Points to the right of the origin are positive numbers, and points to the left are negative numbers. Any number plotted on the line is called a coordinate. In early counting, children may simply learn how to plot coordinates on a line. The line usually is finished on each side with arrows, suggesting the line could go on essentially forever on both the left and the right side.
Sometimes, in early math lessons, the number line is not a line but a line segment or ray. The origin point of zero is the beginning of the line and signified by a dot instead of an arrow pointing to the negative number side. This is usually done to help young children with basic counting skills, since the concept of negative numbers is not introduced until children are in the 4th or 5th grade.
When negative numbers are taught, the number line can be a useful aid since one can simply count coordinates to get to the appropriate answer. For example, if a child is given the problem 57, he can count back seven spaces to arrive at the appropriate answer of 2. Similarly, the problem 3 + 4 can be solved by counting forward four spaces to arrive at the answer of +1. Note this will only work when the coordinates on the number line are whole numbers, or the child knows not to count coordinates that are not whole numbers.
Learning how to use a number line is excellent preparation for learning how to graph numbers. When students must learn how to graph coordinates, their familiarity with the horizontal number line makes it easier to understand vertical ones and the intersection of two numbers that are based on an (x, y) axis. Sometimes in preparation for learning how to find coordinates, students work for a short time with vertical instead of horizontal lines so their familiarity with finding numbers either from left to right and from bottom to top increases.
anon147415 Post 5 
The terms 'standard' and 'customarily used' number line would infer that it would be possible for a number line to have positive values on the left and negative values on the right. 
sneakers41 Post 4 
Cupcake15 He or she may use plastic bears and line up eight in a row and then remove three. Here the children will understand the concept of subtraction much better.
Concrete aids like the number line and use of manipulatives really make a difference when a child is learning math concepts for the first time.
I know that an algebra number line can help kids understand equations that involve greater than and less than equations.
In addition, children will also be able to understand negative numbers. Using number line equations allows children to understand that numbers below zero exist and how we determine that.
Using a money number line is probably more practical because most children have seen money and can relate to the concept. This will also help with understanding negative numbers or the idea of a negative money balance. 
cupcake15 Post 3 
SurfNturfI know that an interactive number line is effect when teaching subtraction for the first time. Young children learning subtraction for the first time can not do mental math yet and have to refer to a number line poster or a classroom number line in order to understand the idea of subtraction.
For example, if a child is given the problem 83=? the child will not be able to mentally make this calculation.
However, if the child has a free number line to look at they can easily see that the answer is 5. A teacher may then move to using manipulatives in the classroom in order to further explain the idea of subtraction. 
surfNturf Post 2 
Frogfriend I agree with you. A classroom number line really helps children learn the abstract concepts of negative numbers and as well as the ability to determine which numbers is between certain numbers.
Children need a number line for subtraction problems and using a number line 1 to 30 really helps.
For example if a child is asked which numbers fall between 15 and 21, in a multiple choice test, many young children will need a number line to determine which values fall within that range.
Children at age 7 begin to conceptualize many abstract math concepts, but some still need concrete visual aids like this in order to find the answer because they are just beginning to develop their mental math skills.

FrogFriend Post 1 
If it wasn't for the number lines that were used in my very first years of education, I don't think I would have fully understood the concepts of math like I do.
The visual aid of being able to count up or down with whatever arithmetic function was key to developing the concept in my mind. 