Of the minerals, or of the grantor's interest?
"1/4 of the minerals" and "1/4 of the grantor's interest" are the same only when the grantor owns all of the minerals.
If the grantor owns 1/2, then 1/4 of the minerals is 1/4, but 1/4 of the grantor's interest is 1/4 × 1/2 = 1/8.
When a deed describes more than the grantor owns, or its wording is unclear, how it is read depends on the whole deed and on state law. This step sticks to clear wording.
A mineral interest or a royalty interest?
A mineral interest owner leases the minerals and is paid the lease royalty on their share. If the lease royalty goes up, so does their decimal.
A royalty interest is a share of production with no right to lease. It can be written two ways:
As a fixed share of production, such as "1/16 of all oil and gas produced." It stays 1/16 whatever the lease says.
As a fraction of the royalty, such as "1/2 of the royalty under any lease." It moves with the lease royalty.
To keep the numbers simple, assume the tract is the whole unit, so each decimal is a share of all production.
"1/2 of the 1/8 royalty"
For decades the usual lease royalty was 1/8, and many older deeds refer to it: "an undivided 1/2 of the 1/8 royalty." This issue generally comes up in older conveyances, written when 1/8 was the standard.
That phrase can be read two ways. It could mean a fixed 1/16 of production (1/2 × 1/8). Or it could mean 1/2 of whatever royalty the lease pays, with "1/8" describing the usual royalty of the time.
Under a 1/8 lease the two readings give the same number. Under a larger royalty they don't.
Courts in different states have read language like this in different ways, and the answer can turn on the rest of the deed. An examiner flags it.
Read the wording
Each phrase below comes from a made-up deed. Decide what its fraction is a fraction of.
Look for the words after the fraction: "of the minerals," "of grantor's interest," "of production," or "of the royalty." A fraction of a fraction, like "1/2 of the 1/8 royalty," is the one that can be read more than one way.
Real deeds are messier than these clean examples. When wording could go more than one way, the result depends on the whole deed and on state law.
States and courts differ on how deed language is read. Check the law where the land is located.