# 'a' speed of / 'an' altitude of

#### Dmitry_86

##### Banned
Dear forum members!!!

Two similar sample sentences:

1) The bullets hit the bus at a speed of 1,350 feet per second
2) The helicopter is flying at an altitude of 1,000 metres

Why is indefinite article used? We indicate the exact velocity and altitude and it seemsto me that "the" should be used. But why do we use "a" and "an"?

Regards

• #### bibliolept

##### Senior Member
It's just customary, I'm afraid; the use of the indefinite is definitely the most idiomatic.

#### Dmitry_86

##### Banned
It's just customary, I'm afraid; the use of the indefinite is definitely the most idiomatic.
I see but we usually use "the" when we mention something particular: (e.g.: in the suburb of Petergoph; for the family living in that house, etc). Why do we use "a/an" here? Is it an unexplainable tradition coming from the past? Or some other logic behind it?

#### Klystron29

##### Senior Member
in the suburb of Petergoph (there is only one suburb)
for the family living in that house (there is only one family)
Whereas the bullet speed varies and the helecopter can fly at different altitudes.

#### Dmitry_86

##### Banned
Whereas the bullet speed varies and the helecopter can fly at different altitudes.
But I have pointed out the exact altitude and speed and I still have not understood why "the" is wrong. If I am to use "a" in such examples like in my very first post I must say:

at a distance of ...
at a speed/velocity of ...
at an acceleration of...
at a/an height/altitude of ...
at a length/width of

So maybe "a/an" is always used with measures?

#### Klystron29

##### Senior Member
Errrrr, no, Dmitry, sorry. As bibliolept pointed out, the indefinite is more idiomatic in the cases where the context is indefinite. I don't want to confuse this issue but let me give you more examples:-
"In that house there is a family from Moscow". In this case the family is vague and unknown and can be any family.
"Yesterday I was invited into that house for welcome drinks by the family from Moscow". In this case we know which family I mean and there is no vagueness about who I am talking.
Does this help Dmitry?
Regards.

#### Dmitry_86

##### Banned
Errrrr, no, Dmitry, sorry. As bibliolept pointed out, the indefinite is more idiomatic in the cases where the context is indefinite. I don't want to confuse this issue but let me give you more examples:-
"In that house there is a family from Moscow". In this case the family is vague and unknown and can be any family.
"Yesterday I was invited into that house for welcome drinks by the family from Moscow". In this case we know which family I mean and there is no vagueness about who I am talking.
Does this help Dmitry?
Regards.
Thanks for the examples!!! Actually the do not help because they are too clear and obvious. In sentence 1 "family" is mentioned for the first time, we do not know what family one is talking about, just a family from Moscow - of course, we use "a". In Example 2 one was invited to the house belonging to a particular family, not to any house in Moscow, and hence we use "the". It is clear!!!

Excuse me for being so insistent, but my examples are diffrent!!! and I still cannot understand them. Let us take one example versus another:

"... at an altitude of ..... "

vs.

"in the suburb of ...."

They seem to me almost the same. I have mentioned a particular suburb in my sentence and used "the", although there are many suburbs (near my city). Equally, I have specified the altitude, there are many possible altitudes for aircraft but I have told which one I am speaking about. So the logic is the same. But why do I use "a/an"?

#### Thomas1

##### Senior Member
Use of a/an
A/an is used
[…]
F. In expressions of price, speed, ratio etc.
5p a kilo
10p a dozen
Ł1 a metre
Four times a day (Here a/an = per)

sixty kilometres an hour

A Practical English Grammar, A.J. Thomson and A.V. Martinet
I realise that the examples given differ from those you gave, Dmitry, but an analogy comes to my mind when I see them and the comment by Bibliolept with the the introductory note in the quotation above.

You may be insterested in this thread where a similar issue was addressed: I want a divorce.

#### Dmitry_86

##### Banned
Use of a/an
A/an is used
[…]
F. In expressions of price, speed, ratio etc.

Any expressions with price, speed, ratio, etc?

#### JulianStuart

##### Senior Member
Perhaps specific numbers are required?

We say : At the speed of sound... , At the speed of light ... and At half the distance to the goal...

In any case one of the definitions of idiom is " makes sense to native speakers but otherwise defies logic" or something like that (OK from the WRDict "an expression whose meanings cannot be inferred from the meanings of the words that make it up")

#### Thomas1

##### Senior Member
Any expressions with price, speed, ratio, etc?
That is a question I can't answer. Language is sometimes something more that a set of hard and fast rules and doesn't always work the way we want it or imagine it. I don't want to sweep generalisations at the risk that someone will come along and throw in a counterexample, but:
1.you see the examples you give (counterexamples including)
2. you know the native's comment on it
2. you know what the book says (there are no changes in the citation)

As an intelligent man you can draw your conclusions.

PS: I've just seen Julian's post.

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