diff --git a/file/error.go b/file/error.go
index c5bd7c4..e7f72e4 100644
--- a/file/error.go
+++ b/file/error.go
@@ -63,6 +63,7 @@ func (err Error) Error () (formattedMessage string) {
 			columnCountdown --
 		}
 		for err.width > 1 {
+			// TODO: for tabs, print out 8 of these instead.
 			formattedMessage += "-"
 		}
 		formattedMessage += "-\n"
diff --git a/lexer/lexer.go b/lexer/lexer.go
index 058e96f..1e543b4 100644
--- a/lexer/lexer.go
+++ b/lexer/lexer.go
@@ -59,8 +59,18 @@ func (lexer *LexingOperation) tokenizeSymbolBeginning () (err error) {
 		}
 	case '\t':
 		// indent level
-		// TODO: throw error if tab is not at line beginning, or after
-		// other tab
+		previousToken := lexer.tokens[len(lexer.tokens) - 1]
+
+		if !previousToken.Is(TokenKindNewline) ||
+			!previousToken.Is(TokenKindNewline) {
+
+			file.NewError (
+				lexer.file.Location(), 1,
+				"tab not used as indent",
+				file.ErrorKindWarn)
+			break
+		}
+		
 		for lexer.char == '\t' {
 			lexer.addToken (Token {
 				kind: TokenKindIndent,
diff --git a/lexer/token.go b/lexer/token.go
index d6d6e90..5454bab 100644
--- a/lexer/token.go
+++ b/lexer/token.go
@@ -62,6 +62,11 @@ func (token Token) Kind () (kind TokenKind) {
 	return token.kind
 }
 
+// Is returns whether or not the token is of kind kind.
+func (token Token) Is (kind TokenKind) (match bool) {
+	return token.kind == kind
+}
+
 // Value returns the value of the token. Depending on what kind of token it is,
 // this value may be nil.
 func (token Token) Value () (value any) {