a, an--Use a before any word or acronym beginning with a consonant
sound. Use an before any word beginning with a vowel sound, regardless
of whether the word begins with a vowel.
a light-water reactor; an LWR; a HEPA
filter (because HEPA is pronounced as a word rather than as letters); a hypothesis;
accuracy, precision--Accuracy is the agreement between the true
value and the result obtained by measurement. Accurate means
"errorless." Precision is the agreement among repeated measurements of
the same quantity. Precise means "clearly delineated, definite."
A sentence can be accurate but not precise. Our
measurements show that the tank contains about 8 gallons (accurate). The
tank contains 8.3 gallons (accurate and precise).
affect, effect--To affect is to influence or change. To effect
is to bring about. An effect is a result or an influence.
Ingesting alcohol affects her driving competence.
Margaritas effect an undesirable change in his personality; he shouts
and tells you that they have no effect on his speech.
NOTE: "Impact" is frequently and
inappropriately used as a verb; "affect" is a better choice. Rather than saying
"These cuts impact our project," say "These cuts affect our
all ready, already--All ready means "completely ready" or
"everyone is ready." Already means "previously."
all right, alright--There is no such word as alright. Forget about
it, all right?
alternate, alternative--To alternate is to occur in successive
turns; first one, then the other. An alternate is a substitute or second
The alternate plan is a good substitute.
An alternative is a choice among mutually exclusive objects or courses
An alternative plan may be one choice of many.
among, between--Among is used with more than two items. Between
is used with two items, except in some close collective relationships.
We quarreled among the five of us, but the real
argument was between Bob and Harold. An alliance was effected between
Germany, Italy, and Japan.
and/or--Use this form only with three selections: one, or the other, or
both. In most writing the distinction is not useful.
assure, ensure, insure--Assure means "to convince," "to
guarantee." Insure means "to guard against loss." Ensure means
"to make certain." Assure and ensure are often used
interchangeably, but they actually have different meanings. Assure takes
a direct object--usually a person or group of persons. You assure someone
that something has been done. Ensure does not imply that you are giving
assurance to someone else. You ensure that something has been done.