diff --git a/src/m_bbox.cpp b/src/m_bbox.cpp
index 6d3a5b7443..199da8d68e 100644
--- a/src/m_bbox.cpp
+++ b/src/m_bbox.cpp
@@ -54,19 +54,8 @@ int FBoundingBox::BoxOnLineSide (const line_t *ld) const
 	int p1;
 	int p2;
 		
-	switch (ld->slopetype)
-	{
-	case ST_HORIZONTAL:
-		p1 = m_Box[BOXTOP] > ld->v1->y;
-		p2 = m_Box[BOXBOTTOM] > ld->v1->y;
-		if (ld->dx < 0)
-		{
-			p1 ^= 1;
-			p2 ^= 1;
-		}
-		break;
-		
-	case ST_VERTICAL:
+	if (ld->dx == 0)
+	{ // ST_VERTICAL
 		p1 = m_Box[BOXRIGHT] < ld->v1->x;
 		p2 = m_Box[BOXLEFT] < ld->v1->x;
 		if (ld->dy < 0)
@@ -74,18 +63,26 @@ int FBoundingBox::BoxOnLineSide (const line_t *ld) const
 			p1 ^= 1;
 			p2 ^= 1;
 		}
-		break;
-		
-	case ST_POSITIVE:
+	}
+	else if (ld->dy == 0)
+	{ // ST_HORIZONTAL:
+		p1 = m_Box[BOXTOP] > ld->v1->y;
+		p2 = m_Box[BOXBOTTOM] > ld->v1->y;
+		if (ld->dx < 0)
+		{
+			p1 ^= 1;
+			p2 ^= 1;
+		}
+	}
+	else if ((ld->dy ^ ld->dx) >= 0)
+	{ // ST_POSITIVE:
 		p1 = P_PointOnLineSide (m_Box[BOXLEFT], m_Box[BOXTOP], ld);
 		p2 = P_PointOnLineSide (m_Box[BOXRIGHT], m_Box[BOXBOTTOM], ld);
-		break;
-		
-	case ST_NEGATIVE:
-	default:	// Just to assure GCC that p1 and p2 really do get initialized
+	}
+	else
+	{ // ST_NEGATIVE:
 		p1 = P_PointOnLineSide (m_Box[BOXRIGHT], m_Box[BOXTOP], ld);
 		p2 = P_PointOnLineSide (m_Box[BOXLEFT], m_Box[BOXBOTTOM], ld);
-		break;
 	}
 
 	return (p1 == p2) ? p1 : -1;
diff --git a/src/p_map.cpp b/src/p_map.cpp
index db0c092021..96b2b61955 100644
--- a/src/p_map.cpp
+++ b/src/p_map.cpp
@@ -2244,24 +2244,8 @@ void FSlide::HitSlideLine (line_t* ld)
 		slidemo->z <= slidemo->floorz &&
 		P_GetFriction (slidemo, NULL) > ORIG_FRICTION;
 
-	if (ld->slopetype == ST_HORIZONTAL)
-	{
-		if (icyfloor && (abs(tmymove) > abs(tmxmove)))
-		{
-			tmxmove /= 2; // absorb half the velocity
-			tmymove = -tmymove/2;
-			if (slidemo->player && slidemo->health > 0 && !(slidemo->player->cheats & CF_PREDICTING))
-			{
-				S_Sound (slidemo, CHAN_VOICE, "*grunt", 1, ATTN_IDLE); // oooff!
-			}
-		}
-		else
-			tmymove = 0; // no more movement in the Y direction
-		return;
-	}
-
-	if (ld->slopetype == ST_VERTICAL)
-	{
+	if (ld->dx == 0)
+	{ // ST_VERTICAL
 		if (icyfloor && (abs(tmxmove) > abs(tmymove)))
 		{
 			tmxmove = -tmxmove/2; // absorb half the velocity
@@ -2276,6 +2260,22 @@ void FSlide::HitSlideLine (line_t* ld)
 		return;
 	}
 
+	if (ld->dy == 0)
+	{ // ST_HORIZONTAL
+		if (icyfloor && (abs(tmymove) > abs(tmxmove)))
+		{
+			tmxmove /= 2; // absorb half the velocity
+			tmymove = -tmymove/2;
+			if (slidemo->player && slidemo->health > 0 && !(slidemo->player->cheats & CF_PREDICTING))
+			{
+				S_Sound (slidemo, CHAN_VOICE, "*grunt", 1, ATTN_IDLE); // oooff!
+			}
+		}
+		else
+			tmymove = 0; // no more movement in the Y direction
+		return;
+	}
+
 	// The wall is angled. Bounce if the angle of approach is		// phares
 	// less than 45 degrees.										// phares
 
diff --git a/src/p_setup.cpp b/src/p_setup.cpp
index f84862ef08..a6dbca868f 100644
--- a/src/p_setup.cpp
+++ b/src/p_setup.cpp
@@ -1888,13 +1888,6 @@ void P_AdjustLine (line_t *ld)
 	ld->dx = v2->x - v1->x;
 	ld->dy = v2->y - v1->y;
 	
-	if (ld->dx == 0)
-		ld->slopetype = ST_VERTICAL;
-	else if (ld->dy == 0)
-		ld->slopetype = ST_HORIZONTAL;
-	else
-		ld->slopetype = ((ld->dy ^ ld->dx) >= 0) ? ST_POSITIVE : ST_NEGATIVE;
-			
 	if (v1->x < v2->x)
 	{
 		ld->bbox[BOXLEFT] = v1->x;
diff --git a/src/po_man.cpp b/src/po_man.cpp
index d844133b08..e19acc53ab 100644
--- a/src/po_man.cpp
+++ b/src/po_man.cpp
@@ -950,18 +950,6 @@ void FPolyObj::UpdateBBox ()
 		// Update the line's slopetype
 		line->dx = line->v2->x - line->v1->x;
 		line->dy = line->v2->y - line->v1->y;
-		if (!line->dx)
-		{
-			line->slopetype = ST_VERTICAL;
-		}
-		else if (!line->dy)
-		{
-			line->slopetype = ST_HORIZONTAL;
-		}
-		else
-		{
-			line->slopetype = ((line->dy ^ line->dx) >= 0) ? ST_POSITIVE : ST_NEGATIVE;
-		}
 	}
 	CalcCenter();
 }
diff --git a/src/r_defs.h b/src/r_defs.h
index 2e9c0876ba..dd21365087 100644
--- a/src/r_defs.h
+++ b/src/r_defs.h
@@ -882,18 +882,6 @@ struct side_t
 
 FArchive &operator<< (FArchive &arc, side_t::part &p);
 
-//
-// Move clipping aid for LineDefs.
-//
-enum slopetype_t
-{
-	ST_HORIZONTAL,
-	ST_VERTICAL,
-	ST_POSITIVE,
-	ST_NEGATIVE
-};
-
-
 struct line_t
 {
 	vertex_t	*v1, *v2;	// vertices, from v1 to v2
@@ -908,7 +896,6 @@ struct line_t
 	side_t		*sidedef[2];
 	//DWORD		sidenum[2];	// sidenum[1] will be NO_SIDE if one sided
 	fixed_t		bbox[4];	// bounding box, for the extent of the LineDef.
-	slopetype_t	slopetype;	// To aid move clipping.
 	sector_t	*frontsector, *backsector;
 	int 		validcount;	// if == validcount, already checked
 	int			locknumber;	// [Dusk] lock number for special