diff --git a/format-specs/geoparquet.md b/format-specs/geoparquet.md
index 2531475..86ae521 100644
--- a/format-specs/geoparquet.md
+++ b/format-specs/geoparquet.md
@@ -227,7 +227,7 @@ Note: This technique to use the bounding box to improve spatial queries does not
 
 ### Bounding Box Columns
 
-A bounding box column MUST be a Parquet group field with 4 child fields named `xmin`, `xmax`, `ymin`, and `ymax` representing the geometry's coordinate range. As with the top-level [`bbox`](#bbox) column, the values follow the GeoJSON specification (RFC 7946, section 5), which also describes how to represent the bbox for geometries that cross the antimeridian. For three dimensions the additional fields `zmin` and `zmax` MAY be present but are not required. The fields MUST be of Parquet type `FLOAT` or `DOUBLE` and all columns MUST use the same type. The repetition of a bounding box column MUST match the geometry column's [repetition](#repetition). A row MUST contain a bounding box value if and only if the row contains a geometry value. In cases where the geometry is optional and a row does not contain a geometry value, the row MUST NOT contain a bounding box value.
+A bounding box column MUST be a Parquet group field with 4 or 6 child fields representing the geometry's coordinate range. For two-dimensional data, the child fields MUST be named `xmin`, `ymin`, `xmax`, and `ymax` and MUST be ordered in this same way. As with the top-level [`bbox`](#bbox) column, the values follow the GeoJSON specification (RFC 7946, section 5), which also describes how to represent the bbox for geometries that cross the antimeridian. For three dimensions the additional fields `zmin` and `zmax` MAY be present but are not required. If `zmin` is present then `zmax` MUST be present and vice versa. If `zmin` and `zmax` are present, the ordering of the child fields MUST be `xmin`, `ymin`, `zmin`, `xmax`, `ymax`, `zmax`. The fields MUST be of Parquet type `FLOAT` or `DOUBLE` and all columns MUST use the same type. The repetition of a bounding box column MUST match the geometry column's [repetition](#repetition). A row MUST contain a bounding box value if and only if the row contains a geometry value. In cases where the geometry is optional and a row does not contain a geometry value, the row MUST NOT contain a bounding box value.
 
 The bounding box column MUST be at the root of the schema. The bounding box column MUST NOT be nested in a group.