altar, alter

Altar or altar means a table or platform used for religious services--
The minister stood at the altar to lead the congregation in prayer.
The church ladies made a fine cloth for the altar.

Alter or alter ( rhymes with altar ) means to change or make different--
The tailor said he could alter your new suit to fit.
The room looks perfect. I wouldn't alter anything.

Now that you know that, you can say--
"We'll have to alter the altar cloth. It's too long."

Note--the noun form ( thing or idea ) of "alter" is "alteration"--
"Love is not love that alters when it alteration finds..."
The tailor shop does alterations, too.
Note # 2--Not to be confused with "alternation", a noun ( thing or idea ) , meaning  switching  back and forth between two different choices--
They argued about the alternation of duties on a weekly basis.
"Alternate" is a verb ( action word--pronounced all ter nate ), meaning to switch between two choices--
Some wanted the duties to alternate on a daily basis.
Manic-depressive people alternate between extremes of happiness and sadness.
An "alternate" ( pronounced all ter net ) is a person who serves in a particular position when the official person can't attend--
The alternate will attend if a juror is absent.
An "alternative" is one of two choices, or a different means or method for accomplishing the same end--
The traffic report said there is a detour. We'll need an alternative route.
"Alternating" is more familiar in the phrase "alternating current"--
Alternating current comes from the electric company, through house wiring or sockets. Direct current comes from batteries.
Note # 3--Not to be confused with "altercation", meaning an argument--
The police stopped by to see about the altercation.