yet another football question

Following a kickoff that is taken for a touchback, the ball is placed on the 20 yard line. So, why is it that the officials would ever need to use the chains to measure a first down on the opening drive? If, following the spot, the nose of the ball is touching the 30 yard line, first down. If not, no first down. Yet I’ve seen several occasions when they will bring out the chains in this situation… WHY?

Well, the simple answer is that on the “opening drive” the first down spots are not necessarily 10 yars apart, for instance if the first play of the drive the play goes for 16.5 yards, the second set of downs are begun from the 36.5 yard line and therefore the chains need to be used.

If what you mean is why are the chains used on the first set of downs, i.e. from the 20 yard line to the 30. The answer is more or less one of procedure. The officials use their discretion when calling for the chains, and to insure that their judgement isn’t flawed they use the chains to be absolutly certain, and to appease the coaches. Chances are that the officials realize that the 30 year line is going to be a first down anyways, but when it comes down to inches the yard lines are not sharply drawn, and may not be reliable. Also the tip of the ball is elevated over the line and the chains act as a T-square to check for plumb. While the accuracy in question here is small compared to the inaccuracy of the initial spot, its a method which tries to make a imperfect procedure consistant. I would argue that the officials use the chains much more rarely in this first 10 yard margin, but on very close calls they use them just to add a air of authority.

Most officials don’t use the chains to measure a first down if the ball started on the 20 after a touchback.
They may use the chains sometimes to spot the ball back at the hash mark at the right spot, although that doesn’t seem particularly efficient.
High school and college refs don’t use the chains in that situation. If they use them that way in the NFL, it’s because: 1) the league makes them do it or 2) one of the teams is requesting a measurement, which is allowed.

Sometimes coaches request a measurement to see just exactly how far they have to go, and sometimes they just want a little more time to discuss strategy. The extra time it takes for a measurement allows them to talk things over with the people in the booth upstairs.

BobT raises a good point, they also use the chains to respot the ball. For example, a running back sweeps to the outside towards the sideline and is tackled a fraction of a yard short in bounds between the hash and the sideline. In this case, the official can see that the ball is short of the first down line, but before the next snap, say 3rd down, the ball must be placed on the hash nearest the spot the runner was tackled. In this case it may be critical that the ball is spotted precisely the same distance short of the first down line. So the official measures with the chains, and then marks the tip of the ball on the chains and carries them to the hash. Here the chains are restreched and the ball is spotted using the officials mark on the chain. Basically its a human drafting table. The chains are important for more than just determining the 10 yard line, they are also used to maintain a consistant spot.