diff --git a/crates/doc/src/config_styles.rs b/crates/doc/src/config_styles.rs
index 44c91ccd..1987f8c9 100644
--- a/crates/doc/src/config_styles.rs
+++ b/crates/doc/src/config_styles.rs
@@ -253,9 +253,20 @@ impl Document {
             return self.certified_pieces(body, indices, target.origin_feature);
         }
         if let Some(super::FaceCertificate::ExtrudeSide { sketch, curve }) = &target.certificate {
-            let indices: Vec<_> = body.body.face_sources().into_iter().flatten().enumerate().filter_map(|(index, source)| {
-                matches!(source, Some(solidcraft_kernel::FaceSource::CertifiedExtrudeSide { producer, sketch: current_sketch, curve: current_curve, .. }) if *producer == target.origin_feature && current_sketch == sketch && current_curve == curve).then_some(index)
-            }).collect();
+            let side = |index: usize, exact: bool| matches!(body.body.face_sources().and_then(|sources| sources.get(index)), Some(Some(solidcraft_kernel::FaceSource::CertifiedExtrudeSide { producer, sketch: current_sketch, curve: current_curve, .. })) if *producer == target.origin_feature && current_sketch == sketch && (!exact || current_curve == curve));
+            let count = body.body.face_sources().map_or(0, |sources| sources.len());
+            let mut indices: Vec<_> = (0..count).filter(|index| side(*index, true)).collect();
+            // A side drawn in collinear pieces is one face over all of them (#1788): a certificate
+            // made when the pieces were faces of their own finds it by the piece's name, which
+            // the joined face answers to.
+            if indices.is_empty() {
+                let names = crate::naming::names(body);
+                indices = (0..count)
+                    .filter(|index| {
+                        side(*index, false) && crate::history_names::matches(&names, *index, &target.name) == Some(crate::history_names::Match::Alias)
+                    })
+                    .collect();
+            }
             if indices.is_empty() {
                 return Err(DocError::Unknown("constructed side certificate no longer exists".into()));
             }
diff --git a/crates/doc/src/eval.rs b/crates/doc/src/eval.rs
index 61f8df0c..d945176c 100644
--- a/crates/doc/src/eval.rs
+++ b/crates/doc/src/eval.rs
@@ -3079,7 +3079,8 @@ fn centered_extrude(plane: &Plane, region: &Region2, start: f64, end: f64, sketc
     };
     // Fit cap surfaces in their own coordinates. Fitting a distant world-space
     // wire can tilt an otherwise exact planar cap through numeric cancellation.
-    let bodies = kernel::extrude(&Plane::XY, std::slice::from_ref(region), start, end)?;
+    // A side drawn in collinear pieces is one face (#1788).
+    let bodies = kernel::extrude_whole_sides(&Plane::XY, std::slice::from_ref(region), start, end)?;
     if *plane == Plane::XY {
         return Ok(bodies.into_iter().map(|body| roles.tool(body, None)).collect());
     }
diff --git a/crates/engine/src/tests.rs b/crates/engine/src/tests.rs
index fe739c29..fcb1b259 100644
--- a/crates/engine/src/tests.rs
+++ b/crates/engine/src/tests.rs
@@ -3693,3 +3693,54 @@ fn pattern_copies_added_after_a_rename_keep_their_numbers() {
     run(&mut s, "timeline.edit", json!({"feature": "Pat", "set": {"pattern": {"count1": "2"}}}));
     assert_eq!(names(&s), ["X", "Second"]);
 }
+
+/// A sketch side drawn as two collinear lines extrudes into one flat face, not two with an edge
+/// between them (#1788); the face is named from the first line and answers to the second.
+#[test]
+fn a_side_drawn_in_two_collinear_pieces_extrudes_into_one_face() {
+    let mut s = Session::default();
+    run(&mut s, "sketch.create", json!({"plane": "XY"}));
+    let r = run(&mut s, "sketch.line", json!({"points": [[0, 0], [10, 0], [20, 0], [20, 10], [0, 10], [0, 0]]}));
+    let pieces: Vec<String> = r["created"]["curves"].as_array().into_iter().flatten().take(2).filter_map(|c| c.as_str().map(String::from)).collect();
+    run(&mut s, "sketch.finish", json!({}));
+    run(&mut s, "solid.extrude", json!({"distance": 5}));
+    let faces = |s: &mut Session| run(s, "model.faces", json!({"body": "Body1"}))["faces"].as_array().cloned().unwrap_or_default();
+    let f = faces(&mut s);
+    assert_eq!(f.len(), 6, "a box has six faces: {f:?}");
+    assert_eq!(run(&mut s, "inspect.measure", json!({}))["bodies"][0]["kernel"]["faces"], 6);
+    let names: Vec<&str> = f.iter().filter_map(|x| x["name"].as_str()).collect();
+    assert!(names.contains(&format!("F2:side:{}", pieces[0]).as_str()), "{names:?}");
+    // The second piece's name answers for the same face, so a reference made when the side was
+    // two faces (an edge of the second piece) still finds its edge.
+    let st = s.model.state();
+    let body = st.body("Body1").unwrap();
+    let edge = format!("F2:side:{}|F2:top", pieces[1]);
+    let (at, _) = solidcraft_doc::naming::find_edge(body, &edge, geom::Vec3::new(15.0, 0.0, 5.0)).unwrap();
+    assert!(at.y.abs() < 1e-9 && (at.z - 5.0).abs() < 1e-9, "{edge} at {at:?}");
+    // A sketch attaches to the joined side as to any side.
+    assert_eq!(run(&mut s, "sketch.create", json!({"plane": {"face": [15, 0, 2.5]}}))["attachment"], "exact_face");
+    run(&mut s, "sketch.finish", json!({}));
+}
+
+/// A sketch made on the second piece's face while the side was two faces (as in a design saved
+/// before #1788) stays attached once the side is straight and extrudes into one face.
+#[test]
+fn a_sketch_on_one_piece_stays_attached_when_the_side_joins() {
+    let mut s = Session::default();
+    run(&mut s, "sketch.create", json!({"plane": "XY"}));
+    let r = run(&mut s, "sketch.line", json!({"points": [[0, 0], [10, -2], [20, 0], [20, 10], [0, 10], [0, 0]]}));
+    let first = r["created"]["curves"][0].as_str().unwrap_or_default().to_string();
+    run(&mut s, "sketch.finish", json!({}));
+    run(&mut s, "solid.extrude", json!({"distance": 5}));
+    let faces = |s: &mut Session| run(s, "model.faces", json!({"body": "Body1"}))["faces"].as_array().map(Vec::len).unwrap_or(0);
+    assert_eq!(faces(&mut s), 7);
+    assert_eq!(run(&mut s, "sketch.create", json!({"plane": {"face": [15, -1, 2.5]}}))["attachment"], "exact_face");
+    run(&mut s, "sketch.circle.center", json!({"center": [5, 2.5], "radius": 1}));
+    run(&mut s, "sketch.finish", json!({}));
+    run(&mut s, "sketch.edit", json!({"sketch": "Sketch1"}));
+    run(&mut s, "sketch.move_point", json!({"point": format!("{first}.end"), "to": [10, 0]}));
+    run(&mut s, "sketch.finish", json!({}));
+    assert_eq!(faces(&mut s), 6, "the side is straight now: one face");
+    let sketch = run(&mut s, "timeline.inspect", json!({"feature": "Sketch2"}));
+    assert!(sketch["error"].is_null() && sketch["warning"].is_null(), "{sketch}");
+}
diff --git a/crates/kernel/src/build.rs b/crates/kernel/src/build.rs
index c6f1d12e..34d0ca77 100644
--- a/crates/kernel/src/build.rs
+++ b/crates/kernel/src/build.rs
@@ -190,6 +190,19 @@ pub(crate) fn frame_ok(plane: &Plane) -> bool {
 
 /// Extrude each region from `start` to `end` along the plane normal (offsets in mm, `end > start`).
 pub fn extrude(plane: &Plane, regions: &[Region2], start: f64, end: f64) -> Result<Vec<Body>> {
+    extrude_with(plane, regions, start, end, false)
+}
+
+/// [`extrude`] with each run of profile lines that continue one another in a straight line (a side
+/// drawn in two pieces, a mirrored half meeting its mirror image) swept into one side face over
+/// all its pieces, rather than coplanar faces with an edge between them (#1788). The side's
+/// history lists every piece under it. For the Extrude feature; other builders (sheet metal's
+/// flat pattern, tools) keep a face per segment.
+pub fn extrude_whole_sides(plane: &Plane, regions: &[Region2], start: f64, end: f64) -> Result<Vec<Body>> {
+    extrude_with(plane, regions, start, end, true)
+}
+
+fn extrude_with(plane: &Plane, regions: &[Region2], start: f64, end: f64, whole_sides: bool) -> Result<Vec<Body>> {
     if !frame_ok(plane) {
         return Err(KernelError::Invalid("extrude frame is not finite and orthonormal".into()));
     }
@@ -206,7 +219,19 @@ pub fn extrude(plane: &Plane, regions: &[Region2], start: f64, end: f64) -> Resu
     for r in regions {
         region_ok(r)?;
         let base = plane.offset(lo);
-        let profile = region_wires(&base, &oriented(r)?)?;
+        let pieces = oriented(r)?;
+        let numbered = if whole_sides { pieces.map_loops(|l| l.merged_lines(continues_straight)) } else { pieces.clone() };
+        // A joined side's source is its first piece, a segment of the profile as drawn (the side
+        // lies along it): the sketch curve it is certified and named by, as when it stood alone.
+        let first_piece = |side: &ProfileEdge| {
+            pieces
+                .loops()
+                .flat_map(|l| l.lp.segs.iter())
+                .copied()
+                .find(|piece| side.numbers.len() > 1 && piece.start() == side.segment.start())
+                .unwrap_or(side.segment)
+        };
+        let profile = region_wires(&base, &numbered)?;
         let wires = wires_of(&profile);
         let f = guard("extrude profile", || builder::try_attach_plane(&wires).map_err(|e| KernelError::Invalid(format!("profile: {e}"))))?;
         let solid: Solid = guard("extrude", || Ok(builder::tsweep(&f, v3(n * (hi - lo)))))?;
@@ -263,7 +288,7 @@ pub fn extrude(plane: &Plane, regions: &[Region2], start: f64, end: f64) -> Resu
                         _ => None,
                     };
                     (
-                        Some(crate::FaceSource::ExtrudeSide { segment: side.segment, plane: basis, coalesced: false }),
+                        Some(crate::FaceSource::ExtrudeSide { segment: first_piece(side), plane: basis, coalesced: false }),
                         Some(record::profile(&side.numbers)),
                     )
                 } else if Some(*id) == end {
@@ -285,6 +310,14 @@ pub fn extrude(plane: &Plane, regions: &[Region2], start: f64, end: f64) -> Resu
     Ok(out)
 }
 
+/// `t` carries on from `s` in the same straight line: the turn at their meeting point is within
+/// rounding of none, measured against both lengths.
+fn continues_straight(s: &Seg2, t: &Seg2) -> bool {
+    let (u, v) = (s.end() - s.start(), t.end() - t.start());
+    let (lu, lv) = (u.len(), v.len());
+    lu > 0.0 && lv > 0.0 && s.end().dist(t.start()) <= 1e-9 * (lu + lv) && u.dot(v) > 0.0 && u.cross(v).abs() <= 1e-9 * lu * lv
+}
+
 /// Choose the per-profile axis sign whose positive rotation follows the sketch normal.
 pub fn canonical_revolve_axis(plane: &Plane, region: &Region2, axis_origin: Vec2, axis_dir: Vec2) -> Result<Vec3> {
     if !axis_origin.is_finite() || !axis_dir.is_finite() || !plane.origin.is_finite() || !plane.x.is_finite() || !plane.y.is_finite() {
diff --git a/crates/kernel/src/history_tests.rs b/crates/kernel/src/history_tests.rs
index ec563f31..25e2a58f 100644
--- a/crates/kernel/src/history_tests.rs
+++ b/crates/kernel/src/history_tests.rs
@@ -244,3 +244,36 @@ fn face_sources_follow_kept_split_and_agreeing_merged_faces() {
     let s = h.face_sources(&[&a]);
     assert_eq!(s, vec![src(0), src(1), None, src(2), None]);
 }
+
+/// A side drawn in two collinear pieces (here the bottom edge, split at x = 10) sweeps into one
+/// side face over both pieces, not two coplanar faces with an edge between them (#1788).
+#[test]
+fn collinear_profile_lines_extrude_into_one_side_face() {
+    use solidcraft_geom::{Loop2, Plane, Region2, Vec2};
+    let pts = [(0.0, 0.0), (10.0, 0.0), (20.0, 0.0), (20.0, 10.0), (0.0, 10.0)].map(|(x, y)| Vec2::new(x, y));
+    let region = Region2 { outer: Loop2::polygon(&pts), holes: vec![] };
+    // Plain `extrude` keeps a face per segment (sheet metal's flat pattern relies on it).
+    assert_eq!(crate::extrude(&Plane::XY, std::slice::from_ref(&region), 0.0, 5.0).unwrap()[0].face_count(), 7);
+    let body = crate::extrude_whole_sides(&Plane::XY, &[region], 0.0, 5.0).unwrap().pop().unwrap();
+    assert_eq!(body.face_count(), 6, "a box has six faces");
+    assert!((crate::measure(&body).unwrap().volume - 1000.0).abs() < 1e-9);
+    let h = body.history().unwrap();
+    h.check(&body, &[]).unwrap();
+    let mut sides: Vec<Vec<FaceOrigin>> =
+        h.faces.iter().map(|f| f.origins.clone()).filter(|o| matches!(o.first(), Some(FaceOrigin::Profile { .. }))).collect();
+    sides.sort();
+    assert_eq!(
+        sides,
+        vec![
+            vec![FaceOrigin::Profile { segment: 0 }, FaceOrigin::Profile { segment: 1 }],
+            vec![FaceOrigin::Profile { segment: 2 }],
+            vec![FaceOrigin::Profile { segment: 3 }],
+            vec![FaceOrigin::Profile { segment: 4 }],
+        ],
+        "the bottom side is generated by both its pieces"
+    );
+    // A corner is not a straight run: an L-shaped step keeps its faces.
+    let step = [(0.0, 0.0), (10.0, 0.0), (10.0, 5.0), (20.0, 5.0), (20.0, 10.0), (0.0, 10.0)].map(|(x, y)| Vec2::new(x, y));
+    let body = crate::extrude_whole_sides(&Plane::XY, &[Region2 { outer: Loop2::polygon(&step), holes: vec![] }], 0.0, 5.0).unwrap().pop().unwrap();
+    assert_eq!(body.face_count(), 8);
+}
diff --git a/crates/kernel/src/lib.rs b/crates/kernel/src/lib.rs
index f29b7dd9..cd85def3 100644
--- a/crates/kernel/src/lib.rs
+++ b/crates/kernel/src/lib.rs
@@ -151,8 +151,8 @@ pub use boundary::{Boundary, boundary_patch};
 pub use boundary_network::{BoundaryCurve, CurveSource, MAX_CHAIN_CURVES, MAX_EDGE_PICKS, boundary_join, boundary_surface};
 pub use bridge::{Continuity, Frame, blend_between, bridge, chain, chain_closes, frames, frames_across, sample_grid};
 pub use build::{
-    PathSeg, box_solid, canonical_revolve_axis, cylinder, extrude, extrude_tapered, extrude_tapered_between, loft, path_polyline, revolve,
-    revolve_whole, sphere, sweep, sweep_with, torus,
+    PathSeg, box_solid, canonical_revolve_axis, cylinder, extrude, extrude_tapered, extrude_tapered_between, extrude_whole_sides, loft,
+    path_polyline, revolve, revolve_whole, sphere, sweep, sweep_with, torus,
 };
 pub use clearance::{Clearance, Occupancy};
 pub use coaxial_overlap::{CoaxialOverlap, coaxial_overlap};