diff --git a/.golangci.yml b/.golangci.yml
index 641471c..ef926a0 100644
--- a/.golangci.yml
+++ b/.golangci.yml
@@ -7,7 +7,6 @@ linters:
     - govet
     - ineffassign
     - staticcheck
-    - typecheck
     - unused
     # disabled by default:
     - gocritic
diff --git a/README.md b/README.md
index 0c96b67..10d549e 100644
--- a/README.md
+++ b/README.md
@@ -21,13 +21,14 @@ which simplifies config management and improves code readability.
 ## 🚀 Features
 
 * Support for all common types and user-defined types
-* Options: [required](#required), [expand](#expand), [slice separator](#slice-separator)
 * Configurable [source](#source) of environment variables
+* Options: [required](#required), [expand](#expand), [slice separator](#slice-separator), [name separator](#name-separator)
 * Auto-generated [usage message](#usage-message)
 
 ## 📦 Install
 
 Go 1.20+
+
 ```shell
 go get go-simpler.org/env
 ```
@@ -38,7 +39,7 @@ go get go-simpler.org/env
 It loads environment variables into the given struct.
 
 The struct fields must have the `env:"VAR"` struct tag,
-where VAR is the name of the corresponding environment variable.
+where `VAR` is the name of the corresponding environment variable.
 Unexported fields are ignored.
 
 ```go
@@ -74,33 +75,35 @@ Nested struct of any depth level are supported,
 allowing grouping of related environment variables.
 
 ```go
-os.Setenv("HTTP_PORT", "8080")
+os.Setenv("DB_HOST", "localhost")
+os.Setenv("DB_PORT", "5432")
 
 var cfg struct {
-    HTTP struct {
-        Port int `env:"HTTP_PORT"`
+    DB struct {
+        Host string `env:"DB_HOST"`
+        Port int    `env:"DB_PORT"`
     }
 }
 if err := env.Load(&cfg, nil); err != nil {
     fmt.Println(err)
 }
 
-fmt.Println(cfg.HTTP.Port) // 8080
+fmt.Println(cfg.DB.Host) // localhost
+fmt.Println(cfg.DB.Port) // 5432
 ```
 
-A nested struct can have the optional `env:"PREFIX"` tag.
-In this case, the environment variables declared by its fields are prefixed with PREFIX.
-This rule is applied recursively to all nested structs.
+If a nested struct has the optional `env:"PREFIX"` tag,
+the environment variables declared by its fields are prefixed with `PREFIX`.
 
 ```go
-os.Setenv("DBHOST", "localhost")
-os.Setenv("DBPORT", "5432")
+os.Setenv("DB_HOST", "localhost")
+os.Setenv("DB_PORT", "5432")
 
 var cfg struct {
     DB struct {
         Host string `env:"HOST"`
         Port int    `env:"PORT"`
-    } `env:"DB"`
+    } `env:"DB_"`
 }
 if err := env.Load(&cfg, nil); err != nil {
     fmt.Println(err)
@@ -112,7 +115,7 @@ fmt.Println(cfg.DB.Port) // 5432
 
 ### Default values
 
-Default values can be specified using the `default:"VALUE"` struct tag:
+Default values can be specified using the `default:"VALUE"` struct tag.
 
 ```go
 os.Unsetenv("PORT")
@@ -167,7 +170,7 @@ fmt.Println(cfg.Addr) // localhost:8080
 ### Slice separator
 
 Space is the default separator used to parse slice values.
-It can be changed with `Options.SliceSep`:
+It can be changed with `Options.SliceSep`.
 
 ```go
 os.Setenv("PORTS", "8080,8081,8082")
@@ -185,7 +188,7 @@ fmt.Println(cfg.Ports) // [8080 8081 8082]
 ### Name separator
 
 By default, environment variable names are concatenated from nested struct tags as is.
-If `Options.NameSep` is not empty, it is used as the separator:
+If `Options.NameSep` is not empty, it is used as the separator.
 
 ```go
 os.Setenv("DB_HOST", "localhost")
@@ -216,7 +219,7 @@ type Source interface {
 }
 ```
 
-Here's an example of using `Map`, a `Source` implementation useful in tests:
+Here's an example of using `Map`, a `Source` implementation useful in tests.
 
 ```go
 m := env.Map{"PORT": "8080"}
@@ -234,7 +237,7 @@ fmt.Println(cfg.Port) // 8080
 ### Usage message
 
 The `Usage` function prints a usage message documenting all defined environment variables.
-An optional usage string can be added to environment variables using the `usage:"STRING"` struct tag:
+An optional usage string can be added to environment variables with the `usage:"STRING"` struct tag.
 
 ```go
 os.Unsetenv("DB_HOST")
@@ -261,7 +264,7 @@ Usage:
   HTTP_PORT  int     default 8080  http server port
 ```
 
-The format of the message can be customized by implementing the `Usage([]env.Var, io.Writer, *env.Options)` method:
+The format of the message can be customized by implementing the `Usage([]env.Var, io.Writer, *env.Options)` method.
 
 ```go
 type Config struct{ ... }
diff --git a/env.go b/env.go
index e0dd0f9..717e31f 100644
--- a/env.go
+++ b/env.go
@@ -15,9 +15,9 @@ type Options struct {
 	NameSep  string // The separator used to concatenate environment variable names from nested struct tags. The default is an empty string.
 }
 
-// NotSetError is returned when environment variables are marked as required but not set.
+// NotSetError is returned when required environment variables are not set.
 type NotSetError struct {
-	Names []string // The names of the missing environment variables.
+	Names []string
 }
 
 // Error implements the error interface.
@@ -51,9 +51,8 @@ func (e *NotSetError) Error() string {
 //
 // Nested struct of any depth level are supported,
 // allowing grouping of related environment variables.
-// A nested struct can have the optional `env:"PREFIX"` tag.
-// In this case, the environment variables declared by its fields are prefixed with PREFIX.
-// This rule is applied recursively to all nested structs.
+// If a nested struct has the optional `env:"PREFIX"` tag,
+// the environment variables declared by its fields are prefixed with PREFIX.
 //
 // Default values can be specified using the `default:"VALUE"` struct tag.
 //
@@ -126,12 +125,11 @@ func parseVars(v reflect.Value, opts *Options) []Var {
 			continue
 		}
 
-		// special case: a nested struct, parse its fields recursively.
+		tags := v.Type().Field(i).Tag
+
 		if kindOf(field, reflect.Struct) && !implements(field, unmarshalerIface) {
 			var prefix string
-			sf := v.Type().Field(i)
-			value, ok := sf.Tag.Lookup("env")
-			if ok {
+			if value, ok := tags.Lookup("env"); ok {
 				prefix = value + opts.NameSep
 			}
 			for _, v := range parseVars(field, opts) {
@@ -141,8 +139,7 @@ func parseVars(v reflect.Value, opts *Options) []Var {
 			continue
 		}
 
-		sf := v.Type().Field(i)
-		value, ok := sf.Tag.Lookup("env")
+		value, ok := tags.Lookup("env")
 		if !ok {
 			continue
 		}
@@ -165,7 +162,7 @@ func parseVars(v reflect.Value, opts *Options) []Var {
 			}
 		}
 
-		defValue, defSet := sf.Tag.Lookup("default")
+		defValue, defSet := tags.Lookup("default")
 		switch {
 		case defSet && required:
 			panic("env: `required` and `default` can't be used simultaneously")
@@ -176,7 +173,7 @@ func parseVars(v reflect.Value, opts *Options) []Var {
 		vars = append(vars, Var{
 			Name:          name,
 			Type:          field.Type(),
-			Usage:         sf.Tag.Get("usage"),
+			Usage:         tags.Get("usage"),
 			Default:       defValue,
 			Required:      required,
 			Expand:        expand,
diff --git a/env_test.go b/env_test.go
index 74a5061..7b596af 100644
--- a/env_test.go
+++ b/env_test.go
@@ -28,7 +28,7 @@ func TestLoad(t *testing.T) {
 			t.Run(name, func(t *testing.T) {
 				const panicMsg = "env: cfg must be a non-nil struct pointer"
 
-				load := func() { _ = env.Load(cfg, &env.Options{Source: env.Map{}}) }
+				load := func() { _ = env.Load(cfg, nil) }
 				assert.Panics[E](t, load, panicMsg)
 
 				usage := func() { env.Usage(cfg, io.Discard, nil) }
@@ -41,7 +41,7 @@ func TestLoad(t *testing.T) {
 		var cfg struct {
 			Foo int `env:""`
 		}
-		load := func() { _ = env.Load(&cfg, &env.Options{Source: env.Map{}}) }
+		load := func() { _ = env.Load(&cfg, nil) }
 		assert.Panics[E](t, load, "env: empty tag name is not allowed")
 	})
 
@@ -49,7 +49,7 @@ func TestLoad(t *testing.T) {
 		var cfg struct {
 			Foo int `env:"FOO,?"`
 		}
-		load := func() { _ = env.Load(&cfg, &env.Options{Source: env.Map{}}) }
+		load := func() { _ = env.Load(&cfg, nil) }
 		assert.Panics[E](t, load, "env: invalid tag option `?`")
 	})
 
@@ -57,7 +57,7 @@ func TestLoad(t *testing.T) {
 		var cfg struct {
 			Foo int `env:"FOO,required" default:"1"`
 		}
-		load := func() { _ = env.Load(&cfg, &env.Options{Source: env.Map{}}) }
+		load := func() { _ = env.Load(&cfg, nil) }
 		assert.Panics[E](t, load, "env: `required` and `default` can't be used simultaneously")
 	})
 
diff --git a/example_test.go b/example_test.go
index aa1f4d9..a586410 100644
--- a/example_test.go
+++ b/example_test.go
@@ -37,30 +37,35 @@ func ExampleLoad_defaultValue() {
 }
 
 func ExampleLoad_nestedStruct() {
-	os.Setenv("HTTP_PORT", "8080")
+	os.Setenv("DB_HOST", "localhost")
+	os.Setenv("DB_PORT", "5432")
 
 	var cfg struct {
-		HTTP struct {
-			Port int `env:"HTTP_PORT"`
+		DB struct {
+			Host string `env:"DB_HOST"`
+			Port int    `env:"DB_PORT"`
 		}
 	}
 	if err := env.Load(&cfg, nil); err != nil {
 		fmt.Println(err)
 	}
 
-	fmt.Println(cfg.HTTP.Port)
-	// Output: 8080
+	fmt.Println(cfg.DB.Host)
+	fmt.Println(cfg.DB.Port)
+	// Output:
+	// localhost
+	// 5432
 }
 
 func ExampleLoad_nestedStructWithPrefix() {
-	os.Setenv("DBHOST", "localhost")
-	os.Setenv("DBPORT", "5432")
+	os.Setenv("DB_HOST", "localhost")
+	os.Setenv("DB_PORT", "5432")
 
 	var cfg struct {
 		DB struct {
 			Host string `env:"HOST"`
 			Port int    `env:"PORT"`
-		} `env:"DB"`
+		} `env:"DB_"`
 	}
 	if err := env.Load(&cfg, nil); err != nil {
 		fmt.Println(err)
diff --git a/usage.go b/usage.go
index 010179a..52ed8c2 100644
--- a/usage.go
+++ b/usage.go
@@ -8,7 +8,7 @@ import (
 )
 
 // cache maps a struct type to the [Var] slice parsed from it.
-// It is primarily needed to fix the following bug:
+// It is needed to fix the following bug:
 //
 //	var cfg struct {
 //		Port int `env:"PORT"`
@@ -16,7 +16,7 @@ import (
 //	env.Load(&cfg, nil)             // 1. sets cfg.Port to 8080
 //	env.Usage(&cfg, os.Stdout, nil) // 2. prints cfg.Port's default == 8080 (instead of 0)
 //
-// It also speeds up [Usage], since there is no need to parse the struct again.
+// It also speeds up [Usage] a bit, since a struct is only parsed once.
 var cache = make(map[reflect.Type][]Var)
 
 // Var holds the information about the environment variable parsed from a struct field.
@@ -34,7 +34,7 @@ type Var struct {
 
 // Usage writes a usage message documenting all defined environment variables to the given [io.Writer].
 // The caller must pass the same [Options] to both [Load] and [Usage], or nil.
-// An optional usage string can be added to environment variables using the `usage:"STRING"` struct tag.
+// An optional usage string can be added to environment variables with the `usage:"STRING"` struct tag.
 // The format of the message can be customized by implementing the Usage([]env.Var, io.Writer, *env.Options) method on the cfg's type.
 func Usage(cfg any, w io.Writer, opts *Options) {
 	pv := reflect.ValueOf(cfg)