1 was the first bus in Chennai from Thiruvanmiyur to Thiruvotriyur
Following this, any bus from Thiruvotriyur to southern Chennai/West via Parrys, Central named as 1 series
Thiruvotriyur to
1A- Thiruvanmiyur
1B-Tambaram
1C,1D-Thiruvanmiyur(Ennore based)
1G-Velachery
1J-Triplicane (this route came after 2011)
101-Poonamalle
Any bus which heads towards Perambur or its localities will be mostly a 29 series.
Perambur to
29A-Marina
29B-Saidapet
29C-Besant Nagar
29E-Thiruverkadu, 29K-CMBT
18 Series-Towards High court/TNagar to Tambaram via Saidapet
18A, E18, B18,18, G18, 18D
17 Series in Broadway to Vadapalani and its localities via Kodambakkam High road
Any bus connecting Broadway with southern Chennai via Marina will be of 21x /x21 series[there is an exception to this]
21G-Tambaram
21H-Kelambakkam
521H-Thiruporur (21H Extn)
21L-Velachery
21E-Iyyapanthangal
C21-Ottiyambakkam
T21-Kannagi Nagar
H21-Chemmenchery
L21-Velachery
PP21-Guduvanchery
S21-Jain College
Exceptions:
Certain x21 series are dedicated for routes from Thiruvanmiyur/Velachery to Tambaram
A21-Tambaram via Pallavaram ring road
R21-Tambaram via Velachery
V21-Guduvanchery via Velachery
Any 27/24/41 series will touch definitely Anna Nagar/Thirumangalam or CMBT and connect to the southern or eastern part of the city
27B-CMBT-Marina
27C-TNagar-CMBT-Thiruverkadu
27D-Villivakkam-Pattinapakkam
27L-JJ Nagar-Marina
27H-Avadi-Marina
24A-Anna Nagar West-V.House
24C-Avadi-V.House
41D-Avadi-Mandaveli
Any 47 series will connect areas around Anna Nagar(ICF/Villivakkam/Ambathur/Avadi) with Adyar/Thiruvanmiyur/Besant Nagar via T.Nagar,Valluvarkottam
All 19 series run in either OMR/ECR
via Adyar/Madhya Kailash
OMR:
19D-Chemmenchery
19K-Siruseri
B19-Shozhinganallur-Kelambakkam
519-Thiruporur
19B-Kelambakkam
ECR:
PP19-Parrys-Kovalam
19H-Kanathur
19G-Kovalam
Any 7 series run from Broadway to localities of Anna Nagar/ICF via Ayanavaram/Doveton
7A,7B,7E,7F,7H,7M(JJ Nagar)
Any 70 series will definitely pass/start thru/from Guindy,Vadapalani,CMBT
70A-Tambaram-Avadi
M70-Thiruvanmiyur
B70,C70-Guindy
D70-Velachery-Ambathur
170-KKD Nagar
570S-Siruseri/Kelambakkam
M70S-Kannagi nagar
60/66 series are mostly routed towards Pallavaram,Pozhichalur,Pammal,Anakaputhur
9,10,11,12(12B),13(triplicane) are miscalleneous routes which definitely touch either T.Nagar/Pondy Bazzar [most congested areas of Chennai]
8,56,44,34,2,4,20 are routes mostly North Chennai buses passing through Washermanpet/Mint
6 series connects Toll gate to Adyar/BN via Parrys – 6A,6D
23 series head towards ICF/Ayanavaram via Saidapet
23C,23V
114 -Redhills-Tambaram/Zoo
54 series towards Extra stage-Guindy-Poonamalle – 54F,54,54B,54L,54K
53 series towards Poonamalle in poonamalle high road
51 series will be mostly heading from Tambaram East/Medavakkam Koot Road Junction
T51-Thiruvanmiyur
C51-Thiruvanmiyur via ECR
A51-Broadway via Saidapet
V51-TNagar via Kilkatalai jn
H51,L51,B51 and many more
PP51Broadway via Adyar
37 series run via Vadapalani,Kodambakkam,Valluvar kottam and extending in both directions
37B,37G
5 series mostly pass through T.Nagar, CIT Nagar, Saidapet and connect to the different localities
5A-Tambaram East
5B-Mylapore
5G-Kannagi Nagar
5E-Besant Nagar-CIT Nagar-Vadapalani
5C-White board bus is an exception
49 series towards Porur
49A-TNagar
49-Iyyapan Thangal-Thiruvanmiyur (recently M49 was changed to 49)
549
42 Series via Periyar Nagar/Vysarpadi from Parrys – 42,242
46 series CMBT to Anna Nagar to Extra Stage
48 series run on ICF Ayanavaram road,Padi bridge – 48,248
59 series run Poonamalle High road and connect CMBT/Purasai -59,159E,159A
15 series run from Broadway to Poonamalle High road connecting Anna Arch,CMBT,MMDA
15B,15F,15G
71 series broadway to Avadi via New Avadi road
120 series run from Broadway to Avadi
M series are appropriately run for reducing the crowd –
Mostly there exists no relationship with above logical ordering
M1-Kilkatalai
M7-TNagar
M5-Kelambakkam
M18M-Vadapalani-Guindy-Tambaram
M15-Medavakkam
M21-Tambaram-Velachery
M23(it was changed to 23M)
M45-Anna Square
M119,M119B-Chemenchery
M19C,M19B-Thuraipakkam
M19A-Kelambakkam
M2A-Mint-Anna Square
M21G-Guindy
Mostly these buses will be white board buses
If it is 5xx /2xx /1xx, mostly it would be an extension from the last two digits.
5xx series covers a longer distance
Live examples:
519,219,219A->19B
521H (21H)
525->25G
254/554->54
549->49
568->68(CMBT-Thiruporur via Guindy,Madhya Kailash)
599(followed from PP19)
Exceptional 3 digit routes:
500/500b/c(follows Chenglepet pattern),502,515,555,587,522,523,597,576,588
Cut services serve 3/4th of the original route or a small extension added to it
ex: E18cut Tambaram-High court.
PS: We spotted this explanation given by Boopal Sridhar in quora.