'the custom that' is a correct and usable part of a sentence in written English.
You can use it when describing a tradition, practice, or behavior that is accepted in a particular society. For example: "The custom that married couples exchange rings as a symbol of their love is an old tradition."
It could require, as was once the custom, that little children riding free sit on a parent's lap.
Monogamy, the custom that allows a person to be legally married to only one spouse at one time.
The result is that the foreign-policy arms of the two parties form a continuum of opinion (excepting, again, the hawks), despite the custom that forbids those who have served in Administrations of one party from serving in Administrations of the other.
One of the earliest forms of lighting fixtures for domestic and public use, sconces first appeared in Classical antiquity, but more elaborate variants were stimulated by the custom that arose in the European Middle Ages of affixing metal sconces holding candles to the walls of churches when they were consecrated.
"The local myth of the lobison is not in any way connected to the custom that began over 100 years ago by which every seventh son (or seventh daughter) born in Argentina becomes godchild to the president," Balmaceda told the Guardian.
"Think of your own church or synagogue or temple, and mosques like this will be very familiar," he said, standing shoeless at the podium in observation of the custom that calls for worshipers to remove their footwear before entering the mosque.
To which of the customs that didn't survive, if any, do you nevertheless see connections to twenty-first century life?
The customs that were adopted were primarily French and were intended to govern and guard a population surrounded by enemies and dependent upon military service to maintain itself.
Most Malawians ignored it in favor of the customs that have governed their lives for as long as anyone can remember.
Risk-avoidance strategies also rely implicitly on the assumption that there are readily available substitutes for local fish and the customs that accompany their gathering (NEJAC 2002).
I'm talking about the customs that arise around intoxicants.
