diff --git a/README.md b/README.md
index 6da36e8..04d6b4b 100644
--- a/README.md
+++ b/README.md
@@ -17,8 +17,8 @@ Development branch. Probably does not work.
 - [x] Benchmark!
 - [x] Abstract AST.
 - [x] Original go-toml testgen tests pass.
-- [ ] Attach comments to AST (gated by parser flag).
 - [ ] Track file position (line, column) for errors.
+- [ ] Attach comments to AST (gated by parser flag).
 - [ ] Benchmark again!
 
 ## Further work
diff --git a/errors.go b/errors.go
new file mode 100644
index 0000000..80db004
--- /dev/null
+++ b/errors.go
@@ -0,0 +1,147 @@
+package toml
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+
+	"github.com/pelletier/go-toml/v2/internal/unsafe"
+)
+
+// DecodeError represents an error encountered during the parsing or decoding
+// of a TOML document.
+//
+// In addition to the error message, it contains the position in the document
+// where it happened, as well as a human-readable representation that shows
+// where the error occurred in the document.
+type DecodeError struct {
+	message string
+	line    int
+	column  int
+
+	human string
+}
+
+// Error returns the error message contained in the DecodeError.
+func (e *DecodeError) Error() string {
+	return e.message
+}
+
+// String returns the human-readable contextualized error. This string is multi-line.
+func (e *DecodeError) String() string {
+	return e.human
+}
+
+/// Position returns the (line, column) pair indicating where the error
+// occurred in the document. Positions are 1-indexed.
+func (e *DecodeError) Position() (row int, column int) {
+	return e.line, e.column
+}
+
+// decodeErrorFromHighlight creates a DecodeError referencing to a highlighted
+// range of bytes from document.
+//
+// highlight needs to be a sub-slice of document, or this function panics.
+//
+// The function copies all bytes used in DecodeError, so that document and
+// highlight can be freely deallocated.
+func decodeErrorFromHighlight(document []byte, highlight []byte, message string) error {
+	err := &DecodeError{
+		message: message,
+	}
+
+	offset := unsafe.SubsliceOffset(document, highlight)
+
+	err.line, err.column = positionAtEnd(document[:offset])
+	before, after := linesOfContext(document, highlight, offset, 3)
+
+	var buf strings.Builder
+
+	maxLine := err.line + len(after) - 1
+	lineColumnWidth := len(strconv.Itoa(maxLine))
+
+	for i := len(before) - 1; i > 0; i-- {
+		line := err.line - i
+		buf.WriteString(formatLineNumber(line, lineColumnWidth))
+		buf.WriteString("| ")
+		buf.Write(before[i])
+		buf.WriteRune('\n')
+	}
+
+	buf.WriteString(formatLineNumber(err.line, lineColumnWidth))
+	buf.WriteString("| ")
+
+	if len(before) > 0 {
+		buf.Write(before[0])
+	}
+	buf.Write(highlight)
+	if len(after) > 0 {
+		buf.Write(after[0])
+	}
+	buf.WriteRune('\n')
+	buf.WriteString(strings.Repeat(" ", lineColumnWidth))
+	buf.WriteString("| ")
+	if len(before) > 0 {
+		buf.WriteString(strings.Repeat(" ", len(before[0])))
+	}
+	buf.WriteString(strings.Repeat("~", len(highlight)))
+	buf.WriteString(" ")
+	buf.WriteString(err.message)
+
+	for i := 1; i < len(after); i++ {
+		buf.WriteRune('\n')
+		line := err.line + i
+		buf.WriteString(formatLineNumber(line, lineColumnWidth))
+		buf.WriteString("| ")
+		buf.Write(after[i])
+	}
+
+	err.human = buf.String()
+	return err
+}
+
+func formatLineNumber(line int, width int) string {
+	format := "%" + strconv.Itoa(width) + "d"
+	return fmt.Sprintf(format, line)
+}
+
+func linesOfContext(document []byte, highlight []byte, offset int, linesAround int) ([][]byte, [][]byte) {
+	var beforeLines [][]byte
+	for beforeOffset, lastOffset := offset, offset; beforeOffset >= 0 && len(beforeLines) <= linesAround; beforeOffset-- {
+		if document[beforeOffset] == '\n' {
+			beforeLines = append(beforeLines, document[beforeOffset+1:lastOffset])
+			lastOffset = beforeOffset
+		} else if beforeOffset == 0 && beforeOffset != lastOffset {
+			beforeLines = append(beforeLines, document[beforeOffset:lastOffset])
+		}
+	}
+
+	var afterLines [][]byte
+
+	document = document[offset+len(highlight):]
+	for afterOffset, lastOffset := 0, 0; afterOffset < len(document) && len(afterLines) <= linesAround; afterOffset++ {
+		if document[afterOffset] == '\n' {
+			afterLines = append(afterLines, document[lastOffset:afterOffset])
+			afterOffset++ // skip \n
+			lastOffset = afterOffset
+		} else if afterOffset == len(document)-1 && lastOffset != afterOffset+1 {
+			afterLines = append(afterLines, document[lastOffset:afterOffset+1])
+		}
+	}
+	return beforeLines, afterLines
+}
+
+func positionAtEnd(b []byte) (row int, column int) {
+	row = 1
+	column = 1
+
+	for _, c := range b {
+		if c == '\n' {
+			row++
+			column = 1
+		} else {
+			column++
+		}
+	}
+	return
+}
diff --git a/errors_test.go b/errors_test.go
new file mode 100644
index 0000000..9d0c5a9
--- /dev/null
+++ b/errors_test.go
@@ -0,0 +1,144 @@
+package toml
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestDecodeError(t *testing.T) {
+	examples := []struct {
+		desc     string
+		doc      [3]string
+		msg      string
+		expected string
+	}{
+		{
+			desc: "no context",
+			doc:  [3]string{"", "morning", ""},
+			msg:  "this is wrong",
+			expected: `
+1| morning
+ | ~~~~~~~ this is wrong`,
+		},
+		{
+			desc: "one line",
+			doc:  [3]string{"good ", "morning", " everyone"},
+			msg:  "this is wrong",
+			expected: `
+1| good morning everyone
+ |      ~~~~~~~ this is wrong`,
+		},
+		{
+			desc: "exactly 3 lines",
+			doc: [3]string{`line1
+line2
+line3
+before `, "highlighted", ` after
+post line 1
+post line 2
+post line 3`},
+			msg: "this is wrong",
+			expected: `
+1| line1
+2| line2
+3| line3
+4| before highlighted after
+ |        ~~~~~~~~~~~ this is wrong
+5| post line 1
+6| post line 2
+7| post line 3`,
+		},
+		{
+			desc: "more than 3 lines",
+			doc: [3]string{`should not be seen1
+should not be seen2
+line1
+line2
+line3
+before `, "highlighted", ` after
+post line 1
+post line 2
+post line 3
+should not be seen3
+should not be seen4`},
+			msg: "this is wrong",
+			expected: `
+3| line1
+4| line2
+5| line3
+6| before highlighted after
+ |        ~~~~~~~~~~~ this is wrong
+7| post line 1
+8| post line 2
+9| post line 3`,
+		},
+		{
+			desc: "more than 10 total lines",
+			doc: [3]string{`should not be seen 0
+should not be seen1
+should not be seen2
+should not be seen3
+line1
+line2
+line3
+before `, "highlighted", ` after
+post line 1
+post line 2
+post line 3
+should not be seen3
+should not be seen4`},
+			msg: "this is wrong",
+			expected: `
+ 5| line1
+ 6| line2
+ 7| line3
+ 8| before highlighted after
+  |        ~~~~~~~~~~~ this is wrong
+ 9| post line 1
+10| post line 2
+11| post line 3`,
+		},
+		{
+			desc: "last line of more than 10",
+			doc: [3]string{`should not be seen 
+should not be seen
+should not be seen
+should not be seen
+should not be seen
+should not be seen
+should not be seen
+line1
+line2
+line3
+before `, "highlighted", ``},
+			msg: "this is wrong",
+			expected: `
+ 8| line1
+ 9| line2
+10| line3
+11| before highlighted
+  |        ~~~~~~~~~~~ this is wrong
+`,
+		},
+	}
+
+	for _, e := range examples {
+		t.Run(e.desc, func(t *testing.T) {
+			b := bytes.Buffer{}
+			b.Write([]byte(e.doc[0]))
+			start := b.Len()
+			b.Write([]byte(e.doc[1]))
+			end := b.Len()
+			b.Write([]byte(e.doc[2]))
+			doc := b.Bytes()
+			hl := doc[start:end]
+
+			err := decodeErrorFromHighlight(doc, hl, e.msg)
+			derr := err.(*DecodeError)
+			assert.Equal(t, strings.Trim(e.expected, "\n"), derr.String())
+		})
+	}
+}
diff --git a/internal/errors/unsafe.go b/internal/unsafe/unsafe.go
similarity index 86%
rename from internal/errors/unsafe.go
rename to internal/unsafe/unsafe.go
index 626c01b..179fd1a 100644
--- a/internal/errors/unsafe.go
+++ b/internal/unsafe/unsafe.go
@@ -1,4 +1,4 @@
-package errors
+package unsafe
 
 import (
 	"fmt"
@@ -8,12 +8,10 @@ import (
 
 const maxInt = uintptr(int(^uint(0) >> 1))
 
-
-func UnsafeSubsliceOffset(data []byte, subslice []byte) int {
+func SubsliceOffset(data []byte, subslice []byte) int {
 	datap := (*reflect.SliceHeader)(unsafe.Pointer(&data))
 	hlp := (*reflect.SliceHeader)(unsafe.Pointer(&subslice))
 
-
 	if hlp.Data < datap.Data {
 		panic(fmt.Errorf("subslice address (%d) is before data address (%d)", hlp.Data, datap.Data))
 	}
@@ -29,7 +27,7 @@ func UnsafeSubsliceOffset(data []byte, subslice []byte) int {
 		panic(fmt.Errorf("slice offset (%d) is farther than data length (%d)", intoffset, datap.Len))
 	}
 
-	if intoffset + hlp.Len > datap.Len {
+	if intoffset+hlp.Len > datap.Len {
 		panic(fmt.Errorf("slice ends (%d+%d) is farther than data length (%d)", intoffset, hlp.Len, datap.Len))
 	}
 
diff --git a/internal/errors/unsafe_test.go b/internal/unsafe/unsafe_test.go
similarity index 83%
rename from internal/errors/unsafe_test.go
rename to internal/unsafe/unsafe_test.go
index 5c3b50b..a184be9 100644
--- a/internal/errors/unsafe_test.go
+++ b/internal/unsafe/unsafe_test.go
@@ -1,4 +1,4 @@
-package errors_test
+package unsafe_test
 
 import (
 	"testing"
@@ -6,13 +6,13 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
-	"github.com/pelletier/go-toml/v2/internal/errors"
+	"github.com/pelletier/go-toml/v2/internal/unsafe"
 )
 
 func TestUnsafeSubsliceOffsetValid(t *testing.T) {
-	examples := []struct{
-		desc string
-		test func() ([]byte, []byte)
+	examples := []struct {
+		desc   string
+		test   func() ([]byte, []byte)
 		offset int
 	}{
 		{
@@ -28,14 +28,14 @@ func TestUnsafeSubsliceOffsetValid(t *testing.T) {
 	for _, e := range examples {
 		t.Run(e.desc, func(t *testing.T) {
 			d, s := e.test()
-			offset := errors.UnsafeSubsliceOffset(d, s)
+			offset := unsafe.SubsliceOffset(d, s)
 			assert.Equal(t, e.offset, offset)
 		})
 	}
 }
 
 func TestUnsafeSubsliceOffsetInvalid(t *testing.T) {
-	examples := []struct{
+	examples := []struct {
 		desc string
 		test func() ([]byte, []byte)
 	}{
@@ -72,7 +72,7 @@ func TestUnsafeSubsliceOffsetInvalid(t *testing.T) {
 		t.Run(e.desc, func(t *testing.T) {
 			d, s := e.test()
 			require.Panics(t, func() {
-				errors.UnsafeSubsliceOffset(d, s)
+				unsafe.SubsliceOffset(d, s)
 			})
 		})
 	}