diff --git a/sharedUI/src/commonMain/kotlin/org/shahondin1624/App.kt b/sharedUI/src/commonMain/kotlin/org/shahondin1624/App.kt
index aed923b..9d5d815 100644
--- a/sharedUI/src/commonMain/kotlin/org/shahondin1624/App.kt
+++ b/sharedUI/src/commonMain/kotlin/org/shahondin1624/App.kt
@@ -9,6 +9,7 @@ import androidx.compose.material.icons.filled.Person
 import androidx.compose.material.icons.filled.Settings
 import androidx.compose.material3.*
 import androidx.compose.runtime.Composable
+import androidx.compose.runtime.CompositionLocalProvider
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.setValue
 import androidx.compose.runtime.rememberCoroutineScope
@@ -23,6 +24,8 @@ import org.shahondin1624.lib.components.charactermodel.attributespage.Attributes
 import org.shahondin1624.model.EXAMPLE_CHARACTER
 import org.shahondin1624.theme.AppTheme
 import org.shahondin1624.theme.LocalThemeIsDark
+import org.shahondin1624.theme.LocalWindowSizeClass
+import org.shahondin1624.theme.WindowSizeClass
 
 @OptIn(ExperimentalMaterial3Api::class)
 @Preview
@@ -30,6 +33,20 @@ import org.shahondin1624.theme.LocalThemeIsDark
 fun App(
     onThemeChanged: @Composable (isDark: Boolean) -> Unit = {}
 ) = AppTheme(onThemeChanged) {
+    BoxWithConstraints(modifier = Modifier.fillMaxSize()) {
+        val windowSizeClass = WindowSizeClass.fromWidth(maxWidth)
+
+        CompositionLocalProvider(LocalWindowSizeClass provides windowSizeClass) {
+            AppContent(onThemeChanged = {})
+        }
+    }
+}
+
+@OptIn(ExperimentalMaterial3Api::class)
+@Composable
+private fun AppContent(
+    onThemeChanged: @Composable (isDark: Boolean) -> Unit = {}
+) {
     val character = EXAMPLE_CHARACTER
     var isDark by LocalThemeIsDark.current
     val drawerState = rememberDrawerState(initialValue = DrawerValue.Closed)
diff --git a/sharedUI/src/commonMain/kotlin/org/shahondin1624/theme/WindowSizeClass.kt b/sharedUI/src/commonMain/kotlin/org/shahondin1624/theme/WindowSizeClass.kt
new file mode 100644
index 0000000..12fc339
--- /dev/null
+++ b/sharedUI/src/commonMain/kotlin/org/shahondin1624/theme/WindowSizeClass.kt
@@ -0,0 +1,36 @@
+package org.shahondin1624.theme
+
+import androidx.compose.runtime.compositionLocalOf
+import androidx.compose.ui.unit.Dp
+import androidx.compose.ui.unit.dp
+
+/**
+ * Classification of window widths following Material Design 3 breakpoints.
+ *
+ * - [Compact]  : width < 600 dp  (phones in portrait)
+ * - [Medium]   : 600 dp <= width <= 840 dp  (tablets, foldables)
+ * - [Expanded] : width > 840 dp  (desktops, landscape tablets)
+ */
+enum class WindowSizeClass {
+    Compact,
+    Medium,
+    Expanded;
+
+    companion object {
+        /**
+         * Returns the [WindowSizeClass] for the given [width].
+         */
+        fun fromWidth(width: Dp): WindowSizeClass = when {
+            width < 600.dp -> Compact
+            width <= 840.dp -> Medium
+            else -> Expanded
+        }
+    }
+}
+
+/**
+ * CompositionLocal providing the current [WindowSizeClass].
+ * Set at the app root via [BoxWithConstraints] so every composable
+ * in the tree can read it without parameter drilling.
+ */
+val LocalWindowSizeClass = compositionLocalOf { WindowSizeClass.Expanded }
diff --git a/sharedUI/src/commonTest/kotlin/org/shahondin1624/WindowSizeClassTest.kt b/sharedUI/src/commonTest/kotlin/org/shahondin1624/WindowSizeClassTest.kt
new file mode 100644
index 0000000..1b9d656
--- /dev/null
+++ b/sharedUI/src/commonTest/kotlin/org/shahondin1624/WindowSizeClassTest.kt
@@ -0,0 +1,42 @@
+package org.shahondin1624
+
+import androidx.compose.ui.unit.dp
+import org.shahondin1624.theme.WindowSizeClass
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class WindowSizeClassTest {
+
+    @Test
+    fun compactForWidthBelow600() {
+        assertEquals(WindowSizeClass.Compact, WindowSizeClass.fromWidth(0.dp))
+        assertEquals(WindowSizeClass.Compact, WindowSizeClass.fromWidth(320.dp))
+        assertEquals(WindowSizeClass.Compact, WindowSizeClass.fromWidth(599.dp))
+    }
+
+    @Test
+    fun mediumForWidth600To840() {
+        assertEquals(WindowSizeClass.Medium, WindowSizeClass.fromWidth(600.dp))
+        assertEquals(WindowSizeClass.Medium, WindowSizeClass.fromWidth(720.dp))
+        assertEquals(WindowSizeClass.Medium, WindowSizeClass.fromWidth(840.dp))
+    }
+
+    @Test
+    fun expandedForWidthAbove840() {
+        assertEquals(WindowSizeClass.Expanded, WindowSizeClass.fromWidth(841.dp))
+        assertEquals(WindowSizeClass.Expanded, WindowSizeClass.fromWidth(1200.dp))
+        assertEquals(WindowSizeClass.Expanded, WindowSizeClass.fromWidth(1920.dp))
+    }
+
+    @Test
+    fun boundaryAt600() {
+        assertEquals(WindowSizeClass.Compact, WindowSizeClass.fromWidth(599.9.dp))
+        assertEquals(WindowSizeClass.Medium, WindowSizeClass.fromWidth(600.dp))
+    }
+
+    @Test
+    fun boundaryAt840() {
+        assertEquals(WindowSizeClass.Medium, WindowSizeClass.fromWidth(840.dp))
+        assertEquals(WindowSizeClass.Expanded, WindowSizeClass.fromWidth(840.1.dp))
+    }
+}