diff --git a/lapack/gonum/dgghrd.go b/lapack/gonum/dgghrd.go
index da09ea9e..c9d6b4d1 100644
--- a/lapack/gonum/dgghrd.go
+++ b/lapack/gonum/dgghrd.go
@@ -10,57 +10,45 @@ import (
 	"gonum.org/v1/gonum/lapack"
 )
 
-// Dgghrd reduces a pair of real matrices (A,B) to generalized upper
-// Hessenberg form using orthogonal transformations, where A is a
-// general matrix and B is upper triangular.  The form of the
-// generalized eigenvalue problem is
+// Dgghrd reduces a pair of real matrices (A,B) to generalized upper Hessenberg
+// form using orthogonal transformations, where A is a general matrix and B is
+// upper triangular.
 //
-//	A*x = lambda*B*x,
+// This subroutine simultaneously reduces A to a Hessenberg matrix H
 //
-// and B is typically made upper triangular by computing its QR
-// factorization and moving the orthogonal matrix Q to the left side
-// of the equation.
-// This subroutine simultaneously reduces A to a Hessenberg matrix H:
+//	Qᵀ*A*Z = H,
 //
-//	Qᵀ*A*Z = H
+// and transforms B to another upper triangular matrix T
 //
-// and transforms B to another upper triangular matrix T:
-//
-//	Qᵀ*B*Z = T
-//
-// in order to reduce the problem to its standard form
-//
-//	H*y = lambda*T*y
-//
-// where y = Zᵀ*x.
+//	Qᵀ*B*Z = T.
 //
 // The orthogonal matrices Q and Z are determined as products of Givens
-// rotations.  They may either be formed explicitly, or they may be
-// postmultiplied into input matrices Q1 and Z1, so that
+// rotations. They may either be formed explicitly (lapack.OrthoExplicit), or
+// they may be postmultiplied into input matrices Q1 and Z1
+// (lapack.OrthoPostmul), so that
 //
-//	Q1 * A * Z1ᵀ = (Q1*Q) * H * (Z1*Z)ᵀ
-//	Q1 * B * Z1ᵀ = (Q1*Q) * T * (Z1*Z)ᵀ
+//	Q1 * A * Z1ᵀ = (Q1*Q) * H * (Z1*Z)ᵀ,
+//	Q1 * B * Z1ᵀ = (Q1*Q) * T * (Z1*Z)ᵀ.
 //
-// If Q1 is the orthogonal matrix from the QR factorization of B in the
-// original equation A*x = lambda*B*x, then Dgghrd reduces the original
-// problem to generalized Hessenberg form.
+// ilo and ihi determine the block of A that will be reduced. It must hold that
+//
+//   - 0 <= ilo <= ihi < n      if n > 0,
+//   - ilo == 0 and ihi == -1   if n == 0,
+//
+// otherwise Dgghrd will panic.
 //
 // Dgghrd is an internal routine. It is exported for testing purposes.
 func (impl Implementation) Dgghrd(compq, compz lapack.OrthoComp, n, ilo, ihi int, a []float64, lda int, b []float64, ldb int, q []float64, ldq int, z []float64, ldz int) {
 	switch {
-	case compq != lapack.OrthoNone && compq != lapack.OrthoEntry && compq != lapack.OrthoUnit:
+	case compq != lapack.OrthoNone && compq != lapack.OrthoExplicit && compq != lapack.OrthoPostmul:
 		panic(badOrthoComp)
-	case compz != lapack.OrthoNone && compz != lapack.OrthoEntry && compz != lapack.OrthoUnit:
+	case compz != lapack.OrthoNone && compz != lapack.OrthoExplicit && compz != lapack.OrthoPostmul:
 		panic(badOrthoComp)
-	case len(a) < (n-1)*lda+n:
-		panic(shortA)
-	case len(b) < (n-1)*ldb+n:
-		panic(shortB)
 	case n < 0:
 		panic(nLT0)
-	case ilo < 0:
+	case ilo < 0 || max(0, n-1) < ilo:
 		panic(badIlo)
-	case ihi < ilo-1 || ihi >= n:
+	case ihi < min(ilo, n-1) || n <= ihi:
 		panic(badIhi)
 	case lda < max(1, n):
 		panic(badLdA)
@@ -70,20 +58,34 @@ func (impl Implementation) Dgghrd(compq, compz lapack.OrthoComp, n, ilo, ihi int
 		panic(badLdQ)
 	case (compz != lapack.OrthoNone && ldz < n) || ldz < 1:
 		panic(badLdZ)
+	}
+
+	// Quick return if possible.
+	if n == 0 {
+		return
+	}
+
+	switch {
+	case len(a) < (n-1)*lda+n:
+		panic(shortA)
+	case len(b) < (n-1)*ldb+n:
+		panic(shortB)
 	case compq != lapack.OrthoNone && len(q) < (n-1)*ldq+n:
 		panic(shortQ)
 	case compz != lapack.OrthoNone && len(z) < (n-1)*ldz+n:
 		panic(shortZ)
 	}
 
-	if compq == lapack.OrthoUnit {
+	if compq == lapack.OrthoExplicit {
 		impl.Dlaset(blas.All, n, n, 0, 1, q, ldq)
 	}
-	if compz == lapack.OrthoUnit {
+	if compz == lapack.OrthoExplicit {
 		impl.Dlaset(blas.All, n, n, 0, 1, z, ldz)
 	}
-	if n <= 1 {
-		return // Quick return if possible.
+
+	// Quick return if possible.
+	if n == 1 {
+		return
 	}
 
 	// Zero out lower triangle of B.
@@ -96,21 +98,19 @@ func (impl Implementation) Dgghrd(compq, compz lapack.OrthoComp, n, ilo, ihi int
 	// Reduce A and B.
 	for jcol := ilo; jcol <= ihi-2; jcol++ {
 		for jrow := ihi; jrow >= jcol+2; jrow-- {
-			// Step 1: rotate rows JROW-1, JROW to kill A(JROW,JCOL).
+			// Step 1: rotate rows jrow-1, jrow to kill A[jrow,jcol].
 			var c, s float64
 			c, s, a[(jrow-1)*lda+jcol] = impl.Dlartg(a[(jrow-1)*lda+jcol], a[jrow*lda+jcol])
 			a[jrow*lda+jcol] = 0
-			bi.Drot(n-jcol-1, a[(jrow-1)*lda+jcol+1:], 1,
-				a[jrow*lda+jcol+1:], 1, c, s)
 
-			bi.Drot(n+2-jrow-1, b[(jrow-1)*ldb+jrow-1:], 1,
-				b[jrow*ldb+jrow-1:], 1, c, s)
+			bi.Drot(n-jcol-1, a[(jrow-1)*lda+jcol+1:], 1, a[jrow*lda+jcol+1:], 1, c, s)
+			bi.Drot(n+2-jrow-1, b[(jrow-1)*ldb+jrow-1:], 1, b[jrow*ldb+jrow-1:], 1, c, s)
 
 			if compq != lapack.OrthoNone {
 				bi.Drot(n, q[jrow-1:], ldq, q[jrow:], ldq, c, s)
 			}
 
-			// Step 2: rotate columns JROW, JROW-1 to kill B(JROW,JROW-1).
+			// Step 2: rotate columns jrow, jrow-1 to kill B[jrow,jrow-1].
 			c, s, b[jrow*ldb+jrow] = impl.Dlartg(b[jrow*ldb+jrow], b[jrow*ldb+jrow-1])
 			b[jrow*ldb+jrow-1] = 0
 
diff --git a/lapack/lapack.go b/lapack/lapack.go
index 72111d66..18790462 100644
--- a/lapack/lapack.go
+++ b/lapack/lapack.go
@@ -231,7 +231,7 @@ const (
 type OrthoComp byte
 
 const (
-	OrthoNone  OrthoComp = 'N' // Do not compute orthogonal matrix.
-	OrthoUnit  OrthoComp = 'I' // Argument is initialized to the unit matrix and the orthogonal matrix is returned.
-	OrthoEntry OrthoComp = 'V' // Argument Q contains orthogonal matrix Q1 on entry and the product Q1*Q is returned.
+	OrthoNone     OrthoComp = 'N' // Do not compute the orthogonal matrix.
+	OrthoExplicit OrthoComp = 'I' // The orthogonal matrix is formed explicitly and returned in the argument.
+	OrthoPostmul  OrthoComp = 'V' // The orthogonal matrix is post-multiplied into the matrix stored in the argument on entry.
 )
diff --git a/lapack/testlapack/dgghrd.go b/lapack/testlapack/dgghrd.go
index 02b0bc86..606ade96 100644
--- a/lapack/testlapack/dgghrd.go
+++ b/lapack/testlapack/dgghrd.go
@@ -5,7 +5,7 @@
 package testlapack
 
 import (
-	"math"
+	"fmt"
 	"testing"
 
 	"golang.org/x/exp/rand"
@@ -20,112 +20,115 @@ type Dgghrder interface {
 }
 
 func DgghrdTest(t *testing.T, impl Dgghrder) {
-	const tol = 1e-13
-	const ldAdd = 5
 	rnd := rand.New(rand.NewSource(1))
-	comps := []lapack.OrthoComp{lapack.OrthoUnit, lapack.OrthoNone, lapack.OrthoEntry}
+	comps := []lapack.OrthoComp{lapack.OrthoExplicit, lapack.OrthoNone, lapack.OrthoPostmul}
 	for _, compq := range comps {
 		for _, compz := range comps {
-			for _, n := range []int{2, 0, 1, 4, 15} {
-				ldMin := max(1, n)
-				for _, lda := range []int{ldMin, ldMin + ldAdd} {
-					for _, ldb := range []int{ldMin, ldMin + ldAdd} {
-						for _, ldq := range []int{ldMin, ldMin + ldAdd} {
-							for _, ldz := range []int{ldMin, ldMin + ldAdd} {
-								testDgghrd(t, impl, rnd, tol, compq, compz, n, 0, n-1, lda, ldb, ldq, ldz)
-							}
-						}
-					}
+			for _, n := range []int{0, 1, 2, 3, 4, 15} {
+				for _, ld := range []int{max(1, n), n + 5} {
+					testDgghrd(t, impl, rnd, compq, compz, n, 0, n-1, ld, ld, ld, ld)
 				}
 			}
 		}
 	}
 }
 
-func testDgghrd(t *testing.T, impl Dgghrder, rnd *rand.Rand, tol float64, compq, compz lapack.OrthoComp, n, ilo, ihi, lda, ldb, ldq, ldz int) {
+func testDgghrd(t *testing.T, impl Dgghrder, rnd *rand.Rand, compq, compz lapack.OrthoComp, n, ilo, ihi, lda, ldb, ldq, ldz int) {
+	const tol = 1e-13
+
 	a := randomGeneral(n, n, lda, rnd)
-	b := blockedUpperTriGeneral(n, n, 0, n, ldb, false, rnd)
-	var q, q1, z, z1 blas64.General
-	if compq == lapack.OrthoEntry {
+	b := randomGeneral(n, n, ldb, rnd)
+
+	var q, q1 blas64.General
+	switch compq {
+	case lapack.OrthoExplicit:
+		// Initialize q to a non-orthogonal matrix, Dgghrd should overwrite it
+		// with an orthogonal Q.
+		q = randomGeneral(n, n, ldq, rnd)
+	case lapack.OrthoPostmul:
+		// Initialize q to an orthogonal matrix Q1, so that the result Q1*Q is
+		// again orthogonal.
 		q = randomOrthogonal(n, rnd)
 		q1 = cloneGeneral(q)
-	} else {
-		q = nanGeneral(n, n, ldq)
 	}
-	if compz == lapack.OrthoEntry {
+
+	var z, z1 blas64.General
+	switch compz {
+	case lapack.OrthoExplicit:
+		z = randomGeneral(n, n, ldz, rnd)
+	case lapack.OrthoPostmul:
 		z = randomOrthogonal(n, rnd)
 		z1 = cloneGeneral(z)
-	} else {
-		z = nanGeneral(n, n, ldz)
 	}
 
 	hGot := cloneGeneral(a)
 	tGot := cloneGeneral(b)
-	for i := 1; i < n; i++ {
-		for j := 0; j < i; j++ {
-			// Set all lower tri elems to NaN to catch bad implementations.
-			tGot.Data[i*tGot.Stride+j] = math.NaN()
-		}
-	}
-	impl.Dgghrd(compq, compz, n, ilo, ihi, hGot.Data, hGot.Stride, tGot.Data, tGot.Stride, q.Data, q.Stride, z.Data, z.Stride)
+	impl.Dgghrd(compq, compz, n, ilo, ihi, hGot.Data, hGot.Stride, tGot.Data, tGot.Stride, q.Data, max(1, q.Stride), z.Data, max(1, z.Stride))
+
 	if n == 0 {
 		return
 	}
+
+	name := fmt.Sprintf("Case compq=%v,compz=%v,n=%v,ilo=%v,ihi=%v", compq, compz, n, ilo, ihi)
+
 	if !isUpperHessenberg(hGot) {
-		t.Error("H is not upper Hessenberg")
-	}
-	if !isNaNFree(tGot) || !isNaNFree(hGot) {
-		t.Error("T or H is/or not NaN free")
+		t.Errorf("%v: H is not upper Hessenberg", name)
 	}
 	if !isUpperTriangular(tGot) {
-		t.Error("T is not upper triangular")
+		t.Errorf("%v: T is not upper triangular", name)
 	}
-	if compq == lapack.OrthoNone {
-		if !isAllNaN(q.Data) {
-			t.Errorf("Q is not NaN")
+	if compq != lapack.OrthoNone {
+		if resid := residualOrthogonal(q, true); resid > tol {
+			t.Errorf("%v: Q is not orthogonal, resid=%v", name, resid)
 		}
-		return
 	}
-	if compz == lapack.OrthoNone {
-		if !isAllNaN(z.Data) {
-			t.Errorf("Z is not NaN")
+	if compz != lapack.OrthoNone {
+		if resid := residualOrthogonal(z, true); resid > tol {
+			t.Errorf("%v: Z is not orthogonal, resid=%v", name, resid)
 		}
-		return
 	}
-	if compq != compz {
-		return // Do not handle mixed case
-	}
-	comp := compq
-	aux := zeros(n, n, n)
 
-	switch comp {
-	case lapack.OrthoUnit:
+	if compq != compz {
+		// Verify reduction only when both Q and Z are computed.
+		return
+	}
+
+	// Zero out the lower triangle of B.
+	for i := 1; i < n; i++ {
+		for j := 0; j < i; j++ {
+			b.Data[i*b.Stride+j] = 0
+		}
+	}
+
+	aux := zeros(n, n, n)
+	switch compq {
+	case lapack.OrthoExplicit:
 		// Qᵀ*A*Z = H
 		hCalc := zeros(n, n, n)
 		blas64.Gemm(blas.Trans, blas.NoTrans, 1, q, a, 0, aux)
 		blas64.Gemm(blas.NoTrans, blas.NoTrans, 1, aux, z, 1, hCalc)
 		if !equalApproxGeneral(hGot, hCalc, tol) {
-			t.Errorf("Qᵀ*A*Z != H")
+			t.Errorf("%v: Qᵀ*A*Z != H", name)
 		}
 
 		// Qᵀ*B*Z = T
 		tCalc := zeros(n, n, n)
 		blas64.Gemm(blas.Trans, blas.NoTrans, 1, q, b, 0, aux)
 		blas64.Gemm(blas.NoTrans, blas.NoTrans, 1, aux, z, 1, tCalc)
-		if !equalApproxGeneral(hGot, hCalc, tol) {
-			t.Errorf("Qᵀ*B*Z != T")
+		if !equalApproxGeneral(tGot, tCalc, tol) {
+			t.Errorf("%v: Qᵀ*B*Z != T", name)
 		}
-	case lapack.OrthoEntry:
+	case lapack.OrthoPostmul:
 		//	Q1 * A * Z1ᵀ = (Q1*Q) * H * (Z1*Z)ᵀ
 		lhs := zeros(n, n, n)
 		blas64.Gemm(blas.NoTrans, blas.NoTrans, 1, q1, a, 0, aux)
-		blas64.Gemm(blas.NoTrans, blas.Trans, 1, aux, z1, 0, lhs) // lhs = Q1 * A * Z1ᵀ
+		blas64.Gemm(blas.NoTrans, blas.Trans, 1, aux, z1, 0, lhs)
 
 		rhs := zeros(n, n, n)
 		blas64.Gemm(blas.NoTrans, blas.NoTrans, 1, q, hGot, 0, aux)
 		blas64.Gemm(blas.NoTrans, blas.Trans, 1, aux, z, 0, rhs)
 		if !equalApproxGeneral(lhs, rhs, tol) {
-			t.Errorf("Q1 * A * Z1ᵀ != (Q1*Q) * H * (Z1*Z)ᵀ")
+			t.Errorf("%v: Q1 * A * Z1ᵀ != (Q1*Q) * H * (Z1*Z)ᵀ", name)
 		}
 
 		//	Q1 * B * Z1ᵀ = (Q1*Q) * T * (Z1*Z)ᵀ
@@ -135,7 +138,7 @@ func testDgghrd(t *testing.T, impl Dgghrder, rnd *rand.Rand, tol float64, compq,
 		blas64.Gemm(blas.NoTrans, blas.NoTrans, 1, q, tGot, 0, aux)
 		blas64.Gemm(blas.NoTrans, blas.Trans, 1, aux, z, 0, rhs)
 		if !equalApproxGeneral(lhs, rhs, tol) {
-			t.Errorf("Q1 * B * Z1ᵀ != (Q1*Q) * T * (Z1*Z)ᵀ")
+			t.Errorf("%v: Q1 * B * Z1ᵀ != (Q1*Q) * T * (Z1*Z)ᵀ", name)
 		}
 	}
 }
diff --git a/lapack/testlapack/general.go b/lapack/testlapack/general.go
index 2291dcab..fb16a4d8 100644
--- a/lapack/testlapack/general.go
+++ b/lapack/testlapack/general.go
@@ -1198,23 +1198,13 @@ func isUpperHessenberg(h blas64.General) bool {
 
 // isUpperTriangular returns whether a contains only zeros below the diagonal.
 func isUpperTriangular(a blas64.General) bool {
+	if a.Rows != a.Cols {
+		panic("matrix not square")
+	}
 	n := a.Rows
 	for i := 1; i < n; i++ {
 		for j := 0; j < i; j++ {
-			v := a.Data[i*a.Stride+j]
-			if v != 0 || math.IsNaN(v) {
-				return false
-			}
-		}
-	}
-	return true
-}
-
-// isNaNFree returns whether a does not contain NaN elements in reachable elements.
-func isNaNFree(a blas64.General) bool {
-	for i := 0; i < a.Rows; i++ {
-		for j := 0; j < a.Cols; j++ {
-			if math.IsNaN(a.Data[i*a.Stride+j]) {
+			if a.Data[i*a.Stride+j] != 0 {
 				return false
 			}
 		}