Is the 18 yard line part of the penalty area?
The penalty area or 18-yard box (also known less formally as the penalty box or simply box) is an area of an association football pitch. It is rectangular and extends 16.5m (18 yd) to each side of the goal and 16.5m (18 yd) in front of it.
The penalty area
The area bounded by these lines and the goal-line is the penalty area. Within each penalty area, a penalty mark is made 11m (12yds) from the midpoint between the goalposts. An arc of a circle with a radius of 9.15m (10yds) from the centre of each penalty mark is drawn outside the penalty area.
Within each penalty area, a penalty mark is made 11m from the midpoint between the goalposts and equidistant to them.
Penalty area, otherwise known as the 18-yard box - this measurement is taken 18 yards from each goal post and 18 yards out from the goal line. Penalty spot - within the penalty area is a spot marked 12 yards in front of the centre of the goal.
Purpose of Rule: Rule 17 is a specific Rule for penalty areas, which are bodies of water or other areas defined by the Committee where a ball is often lost or unable to be played. For one penalty stroke, you may use specific relief options to play a ball from outside the penalty area.
Answer: It's for penalties, when players have to be at least 10-yards from the penalty spot. That semi-circle marks exactly 10 yards from the spot.
Penalty areas are one of the five defined areas of the course and can be marked as either red or yellow. When your ball lies in a penalty area, you can play it as it lies or take relief outside the penalty area for one penalty stroke.
The player taking the penalty kick must kick the ball forward; backheeling is permitted provided the ball moves forward. When the ball is kicked, the defending goalkeeper must have at least part of one foot touching, in line with, or behind, the goal line.
The basket is 1.2 meters, or almost four feet, in from the baseline, which puts the foul line at 4.6 meters — 15 feet — away. The restricted area under the basket is 1.25 meters or just a shade more than four feet in radius.
A goal is scored when the whole of the ball passes over the goal line, between the goalposts and under the crossbar, provided that no offence has been committed by the team scoring the goal.
How big is an 18-yard box in soccer?
18 Yard Box
The box is about 44 yards wide and 18 yards deep. It also has an arc that can be spotted 10 yards from the penalty mark. In the 18 yard box the goalkeeper is the only player that has permission to grip the ball. You should also avoid slide tackles in this area as they could easily result in a penalty kick.
The length (touch line) has to be minimum 90 metres (100 yds) and maximum 120 metres (130 yds). The width (goal line) has to be minimum 45 metres (50 yds) and maximum 90 metres (100 yds).

Double the distance takes us to 12 yards. Add another 6 yards on and you've got 18 yards, which is the distance from the goal line that the edge of the area runs. The area also starts 18 yards from either post, which gives us the rectangular penalty area.
Normally 5 - 6 steps away from the ball will do when taking a penalty.
Goal Area - The goal area is a box that extends 6 yards out from the goal posts. Free kicks are taken from this area. Penalty Area - The penalty area is a box that extends 18 yards out from the goal posts. In this area the goalkeeper may use their hands.
When a player's ball is in a penalty area, there is no relief for: Interference by an abnormal course condition (Rule 16.1), An embedded ball (Rule 16.3), or. An unplayable ball (Rule 19).
Rule 17.1d allows a player to choose a reference point on the reference line that determines the relief area. Each relief Rule requires the player to use a specific relief area whose size and location are based on these three factors:(... Continued) for back-on-the-line relief.
If a ball is lost or out of bounds, the player must take stroke-and-distance relief by adding one penalty stroke and playing the original ball or another ball from where the previous stroke was made (see Rule 14.6).
The goal area (colloquially the "six-yard box"), consists of the rectangle formed by the goal-line, two lines starting on the goal-line 5.5 metres (6 yd) from the goalposts and extending 5.5 metres (6 yd) into the pitch from the goal-line, and the line joining these, i.e. they are a rectangle 6yds by 20yds.
The goalkeeper is just like any other player, except when he/she is inside the penalty box. The number one main difference is that inside the penalty box the goalkeeper can touch the ball with any part of their body, most importantly their hands.
Can a goalkeeper handle the ball in the penalty arc?
The goalkeeper has the same restrictions on handling the ball as any other player outside the penalty area. If the goalkeeper handles the ball inside their penalty area when not permitted to do so, an indirect free kick is awarded but there is no disciplinary sanction.
Every other player must be outside the area, and at least 10 yards from the ball. The 'D'marks the part of the pitch outside the penalty box which is closer to the penalty spot than 10 yards, into which players must not encroach before the kick. It is usually called the D.
The total of rushing yards and passing yards (but not receiving yards) is known as total offense. Although the ball may also be advanced by penalty, these yards do not contribute to total offense.